Protection and treatment of hypothermia in ... - Umeå universitet

In prehospital trauma care active warming is recommended to aid in protection
from further cooling. However, scientific evidence of the effectiveness of active warming in a
clinical setting is scarce. Also, evaluating the effectiveness of active warming, especially in
harsh ambient conditions, by objective measures, is difficult.
To evaluate the effectiveness of field applicabe heat sources (I) and to evaluate
active warming intervention in a prehospital clinical setting (II and III).
To evaluate reliability and validity of the Cold Discomfort Scale (CDS), a subjective
judgement scale for assessment of the thermal state of patients in a cold environment (IV).
In a laboratory trial, non-shivering hypothermic subjects (n=5), were cooled in 8
ºC water followed by spontaneous warming, a charcoal heater, two flexible hot-water bags or
two chemical heat pads, all applied to the chest and upper back (I). Oesophageal temperature,
skin temperature, heat flux, oxygen consumption, respiratory rate and, heart rate were
measured.
In two clinical randomized trials, shivering patients during road and air ambulance transport
(II) and during field treatment (III) were randomized to either passive warming alone (n=22
and n=9) or to passive warming with the addition of a chemical heat pad (n=26 and n=11).
Body core temperature, respiratory rate, heart rate, blood pressure (II) and the patients’
subjective sensation of thermal comfort (II and III) were measured.
In a laboratory trial, shivering subjects were exposed to – 20 ºC (n=22). The CDS was
evaluated regarding reliability, defined as test-retest stability, and criterion validity, defined as
the ability to detect changes in cold discomfort due to changes in cumulative cold stress (IV).
In non-shivering hypothermic subjects postcooling afterdrop was significantly less
for the chemical heat pads, but not for the hot water bags and the charcoal heater, compared to
spontaneous warming (I). Temperature drop during the entire warming phase was
significantly less for all the heat sources respectively, compared to spontaneous warming (I).
During road and air ambulance transport, ear canal temperature was significantly increased
and cold discomfort significantly decreased, both in patients assigned to passive warming
only, and in patients assigned to additional active warming (II). During field treatment, cold
discomfort was significantly reduced in patients assigned to additional active warming, but
remained the same in patients assigned to passive warming only (III).
Weighted kappa coefficient, describing test-retest stability, was 0.84 (IV). CDS ratings were
significantly increased during each 30 minutes interval (IV).
In non-shivering hypothermic subjects, heat sources were effective to attenuate
afterdrop, when providing high heat content over a large surface area and effective to
continue to increase body core temperature when providing sustained high heat content. In
shivering trauma patients, adequate passive warming were sufficient treatment to prevent
afterdrop, to slowly increase body core temperature, and to reduce cold discomfort. If
inadequate passive warming, additional active warming was required to reduce cold
discomfort. The CDS, a subjective judgement scale for assessment of the thermal state of
patients in a cold environment seemed to be reliable regarding test-retest stability and valid
regarding ability to detect change in cumulative cold stress.

This thesis is based on the following studies, which will be referred to in the text by
their Roman numerals:
I Lundgren JP, Henriksson O, Pretorius T, Cahill F, Bristow G, Chochinov A,
Pretorius A, Bjornstig U, Giesbrecht GG. Field Torso Warming Modalities: A
Comparative Study Using a Human Model. Prehosp Emerg Care 2009,3:371-
378.
II Lundgren P, Henriksson O, Naredi P, Bjornstig U. The effect of active
warming in prehospital trauma care during road and air ambulance
transportation - a clinical randomized trial. Scandinavian Journal of Trauma,
Resuscitation and Emergency Medicine 2011 19:59.
III Lundgren P, Henriksson O, Naredi P, Bjornstig U. The Effect of Active
Warming on Cold Discomfort in Field Treatment of Trauma Patients – a
Clinical Randomized Trial. Manuscript.
IV Lundgren P, Henriksson O, Kuklane K, Holmér I, Naredi P, Bjornstig U.
Validity and Reliability of the Cold Discomfort Scale - a Subjective Judgement
Scale for Assesssment of the Thermal State of Patients in a Cold Environment.
Manuscript.
Original papers were reprinted with approval from the publishers.

En skadad person löper stor risk att bli nedkyld, men redan innan allmän
nedkylning är ett faktum, aktiveras kroppens försvarsmekanismer i form av
sammandragning av ytliga blodlärl för att minska värmeförlusterna samt huttring för
att öka värmeproduktionen. Detta innebär, förutom ett ökat obehag, en stor
fysiologisk belastning för kroppen som kan vara ödesdiger i kombination med andra
yttre skador. Om allmän nedkylning ändå inträffar medför detta ökad
blödningsbenägenhet, vätskeförluster och risk att drabbas av livshotande
hjärtrytmrubbningar. Allmän nedkylning i samband med allvarliga kroppsskador har
visat sig vara en negativ prognostisk faktor associerad med ökad dödlighet. Vid
omhändertagande på skadeplats samt under transport in till sjukhus är det således av
yttersta vikt att skydda skadade patienter mot ytterligare nedkylning och åtgärder för
att förhindra detta är prioriterade. Isolering mot yttre påverkan av väta, blåst och
kyla utgör grunden för behandling. Därtill rekommenderas i såväl nationella och
internationella riktlinjer att någon form av extern värmekälla ska tillföras patienten
redan på skadeplats och under transport som komplement till isolering i syfte att
minska den fysiologiska belastningen på kroppen.
De förhållanden som råder utanför sjukhus sätter begränsningar avseende
möjligheter att adekvat mäta hur kylan påverkar patienten. Det är svårt att med
tillförlitliga medel mäta central kroppstemperatur på skadeplats eller under transport
til sjuhus och det är än svårare att under dessa omständighetrer objektivt mäta
graden av huttring.
Utvärdering av extern värmetillförsel på skadeplats samt under transport,
avseende inverkan på fysiologisk samt även psykologisk belastning på kroppen.
Specifikt var syftet att, under kontrollerade former i laboratorium, utvärdera och
jämföra effekt av fältmässigt anpassad materiel för värmetillförsel (studie I) och att i
kliniska situationer (studie II och III) utvärdera utvärdera effekten av sådan materiel,
som tillägg till ordinarie behandling med isolering. Utöver detta var syftet även att
utvärdera en alternativ mätmetod i form av en skala för subjektiv skattning av
upplevelse av kyla, Cold Discomfort Scale (CDS), i syfte att kunna bedöma graden
av kylapåverkan redan tidigt i förloppet.
I ett laboratorium exponerades forskningspersoner (n = 5) för kyla i form av
8 ºC vatten, för att därefter erhålla behandling med en varm sovsäck och en av fyra
olika behandlingsmetoder; en kolbricketdriven värmare, kemiska värmekuddar,
varmvattensäckar, alternativt ingen värmebehandling alls (studie I).
Försökspersonernas huttring hämmades med läkemedel. Central kroppstemperatur,
värmeöverföring, syrgasförbrukning som ett mått på huttring samt andnings- och
hjärtfrekvens mättes
I två kliniska studier av behandling på skadeplats (studie II) och behandling under
transport till sjukhus (studie III) randomiserades lindrigt skadade patienter till
antingen ordinarie behandling med endast isolering (n = 22 och n = 9) eller till
behandling ordinarie med tillägg av extern värmekälla (n = 26 och n = 11).

Kroppstemperatur, andningsfrekvens, hjärtfrekvens och blodtryck (studie II) samt
patienternas upplevelse av kyla (studie III) mättes.
Forskningspersoner (n = 22) exponerades vid två upprepade tillfällen med identiska
förutsättningar för - 20 ºC i 60 minuter. CDS utvärderades för reliabilitet avseende testretest
stabilitet och för kriterievaliditet, definierad som förmåga att mäta skillnad i
kumulativ kylapåverkan.
Under förhållanden där forskningspersonernas huttring hämmades med
läkemedel sjönk kroppstemperaturen mindre, avseende lägsta uppmätta
kroppstemperatur (afterdrop), vid behandling med de kemiska värmekuddarna, men
inte vid behandling med varmvattensäckar eller kolbrickettdriven värmare, jämfört
med ingen värmebehandling alls. Under hela behandlingsfasen, sjönk
kroppstemperaturen mindre vid behandling med såväl de kemiska värmekuddarna,
varmvattensäckarna som den kolbricketdrivna värmaren.
Under transport till sjukhus ökade patienternas centrala kroppstemperatur, samtidigt
som deras upplevelse av kyla minskade signifikant både vid behandling med endast
ordinarie isolering och vid behandling med tillägg av extern värmekälla (studie II).
På skadeplats och under inledande transport utomhus minskade patienternas
upplevelse av kyla signifikant vid behandling med tillägg av extern värmekälla till
ordinarie isolering, men den kvarstod däremot oförändrad vid behandling med
endast isolering.
Viktad kappa koefficient, som mått på test-retest stabilitet, var 0.84. CDS visade
signifikant ökad niva för upplevelse av kyla vid jämförelse av mätningar med 30
minuters mellanrum.
Externa värmekällor som tillförde mycket värmeenergi över en stor
kontaktyta var effektiva för att minska afterdrop och under förutsättning att de tillför
denna värmeenergi under lång tid, även effektiva för att påbörja uppvärmning vid
behandling av patienter som har nedsatt huttringsförmåga. Avseende patienter som
har bibehållen huttringsförmåga var adekvat mängd isolering tillräckligt för att
motverka afterdrop, påbörja uppvärmning, och minska upplevelsen av kyla, men då
isoleringen, utifrån rådande omgivningsförhållanden, inte var adekvat krävdes
tillägg av extern värmekälla för att minska upplevelsen av kyla. Subjektiv skattning
av upplevelse av kyla enligt CDS uppvisade både god reliabilitet avseende test-retest
stabilitet och god validitet avseende förmåga att påvisa skillnad i kumulativ
kylapåverkan.

Kroppstemperatur, andningsfrekvens, hjärtfrekvens och blodtryck (studie II) samt
patienternas upplevelse av kyla (studie III) mättes.
Forskningspersoner (n = 22) exponerades vid två upprepade tillfällen med identiska
förutsättningar för - 20 ºC i 60 minuter. CDS utvärderades för reliabilitet avseende testretest
stabilitet och för kriterievaliditet, definierad som förmåga att mäta skillnad i
kumulativ kylapåverkan.
Under förhållanden där forskningspersonernas huttring hämmades med
läkemedel sjönk kroppstemperaturen mindre, avseende lägsta uppmätta
kroppstemperatur (afterdrop), vid behandling med de kemiska värmekuddarna, men
inte vid behandling med varmvattensäckar eller kolbrickettdriven värmare, jämfört
med ingen värmebehandling alls. Under hela behandlingsfasen, sjönk
kroppstemperaturen mindre vid behandling med såväl de kemiska värmekuddarna,
varmvattensäckarna som den kolbricketdrivna värmaren.
Under transport till sjukhus ökade patienternas centrala kroppstemperatur, samtidigt
som deras upplevelse av kyla minskade signifikant både vid behandling med endast
ordinarie isolering och vid behandling med tillägg av extern värmekälla (studie II).
På skadeplats och under inledande transport utomhus minskade patienternas
upplevelse av kyla signifikant vid behandling med tillägg av extern värmekälla till
ordinarie isolering, men den kvarstod däremot oförändrad vid behandling med
endast isolering.
Viktad kappa koefficient, som mått på test-retest stabilitet, var 0.84. CDS visade
signifikant ökad niva för upplevelse av kyla vid jämförelse av mätningar med 30
minuters mellanrum.
Externa värmekällor som tillförde mycket värmeenergi över en stor
kontaktyta var effektiva för att minska afterdrop och under förutsättning att de tillför
denna värmeenergi under lång tid, även effektiva för att påbörja uppvärmning vid
behandling av patienter som har nedsatt huttringsförmåga. Avseende patienter som
har bibehållen huttringsförmåga var adekvat mängd isolering tillräckligt för att
motverka afterdrop, påbörja uppvärmning, och minska upplevelsen av kyla, men då
isoleringen, utifrån rådande omgivningsförhållanden, inte var adekvat krävdes
tillägg av extern värmekälla för att minska upplevelsen av kyla. Subjektiv skattning
av upplevelse av kyla enligt CDS uppvisade både god reliabilitet avseende test-retest
stabilitet och god validitet avseende förmåga att påvisa skillnad i kumulativ
kylapåverkan.

Kroppstemperatur, andningsfrekvens, hjärtfrekvens och blodtryck (studie II) samt
patienternas upplevelse av kyla (studie III) mättes.
Forskningspersoner (n = 22) exponerades vid två upprepade tillfällen med identiska
förutsättningar för - 20 ºC i 60 minuter. CDS utvärderades för reliabilitet avseende testretest
stabilitet och för kriterievaliditet, definierad som förmåga att mäta skillnad i
kumulativ kylapåverkan.
Under förhållanden där forskningspersonernas huttring hämmades med
läkemedel sjönk kroppstemperaturen mindre, avseende lägsta uppmätta
kroppstemperatur (afterdrop), vid behandling med de kemiska värmekuddarna, men
inte vid behandling med varmvattensäckar eller kolbrickettdriven värmare, jämfört
med ingen värmebehandling alls. Under hela behandlingsfasen, sjönk
kroppstemperaturen mindre vid behandling med såväl de kemiska värmekuddarna,
varmvattensäckarna som den kolbricketdrivna värmaren.
Under transport till sjukhus ökade patienternas centrala kroppstemperatur, samtidigt
som deras upplevelse av kyla minskade signifikant både vid behandling med endast
ordinarie isolering och vid behandling med tillägg av extern värmekälla (studie II).
På skadeplats och under inledande transport utomhus minskade patienternas
upplevelse av kyla signifikant vid behandling med tillägg av extern värmekälla till
ordinarie isolering, men den kvarstod däremot oförändrad vid behandling med
endast isolering.
Viktad kappa koefficient, som mått på test-retest stabilitet, var 0.84. CDS visade
signifikant ökad niva för upplevelse av kyla vid jämförelse av mätningar med 30
minuters mellanrum.
Externa värmekällor som tillförde mycket värmeenergi över en stor
kontaktyta var effektiva för att minska afterdrop och under förutsättning att de tillför
denna värmeenergi under lång tid, även effektiva för att påbörja uppvärmning vid
behandling av patienter som har nedsatt huttringsförmåga. Avseende patienter som
har bibehållen huttringsförmåga var adekvat mängd isolering tillräckligt för att
motverka afterdrop, påbörja uppvärmning, och minska upplevelsen av kyla, men då
isoleringen, utifrån rådande omgivningsförhållanden, inte var adekvat krävdes
tillägg av extern värmekälla för att minska upplevelsen av kyla. Subjektiv skattning
av upplevelse av kyla enligt CDS uppvisade både god reliabilitet avseende test-retest
stabilitet och god validitet avseende förmåga att påvisa skillnad i kumulativ
kylapåverkan.

ASHRAE American Society of Heating, Refrigerating, and Air-
Conditioning Engineers
ANOVA Analysis of Variance
C Celsius
CDS Cold Discomfort Scale
CI Confidence interval
EMS Emergency Medical Services
ICAR International Commission for Alpine Rescue
IQR Interquartile range
NRS Numerical Rating Scale
PLSD Protected Least Significant Differences
SaR Search and Rescue
SD Standard deviation
VAS Visual Analoude Scale
VRS Verbal Rating Scale

In a prehospital rescue scenario, cold exposure poses a considerable risk for injured
or ill patients. Admission hypothermia is associated with worse outcome and higher
mortality in trauma patients (1-8). The cold induced stress response will also render
considerable thermal discomfort which might increase the experience of pain and
anxiety, even in still normothermic patients (9). In Sweden the average temperature
in January during the period 1961 – 90 were between – 16 ºC in the northern part
and – 1 ºC in the southern part of the country (10). In such environment, protection
and treatment of hypothermia in prehospital trauma care is vitally important.
During the period 2001 – 2010, the median annual incidence of fatal accidental
hypothermia in Sweden (about 9,5 million inhabitants) was 48 (range 37 – 63) (11).
Hypothermia due to cold exposure as the primary cause (primary hypothermia)
mainly affects two groups of patients; one group of elderly, most of them chronic
abusers and under the influence of alcohol, and another group of younger, sober
persons, performing outdoor leisure and sporting activities (12).
Although primary hypothermia is a rare diagnosis, secondary hypothermia, as a
complication of systemic disorders, including trauma and sepsis, is common. In
trauma patients, the overall medical condition and the administration of analgesic or
anaesthetic drugs act to impair thermoregulatory mechanisms (13-16). In addition,
wet clothing, contact with cold surfaces, large bleedings and administration of cold
intravenous fluids contribute to the cold stress. Reported hypothermia incidence in
the prehospital setting or upon arrival to hospital varies over a wide range (1, 4-8,
17-19). Severity of injury, the presence of haemorrhagic shock, prehospital
induction of anaesthesia and protracted evacuation are predictive variables of
admission hypothermia (1-5, 18, 20-22), reported hypothermia incidence is therefore
dependent on inclusion criteria of the trauma registers and also dependent on
whether hypothermia is being defined as body core temperature < 35 ºC or < 36 ºC.
In one retrospective study using the US National Trauma Data Bank including more
than 700000 patients, Martin et al. (4) found an hypothermia (< 35 ºC) incidence of
about 2 %, whereas Helm et al (18), in a prospective study including 302 trauma
patients treated during primary helicopter rescue missions, found a hypothermia (<
36 ºC) incidence of about 50%.
Induced hypothermia diminishes complications due to ischemia reperfusion
injury during elective surgery (23). In laboratory studies, hypothermia has been
shown to have beneficial effects during and after hemorrhagic shock and traumatic
brain injury (23-25). The reduced physiological stress due to shivering prevention in
induced hypothermia, compared to the increased physiological stress in accidental
hypothermia, has been suggested a reason for these beneficial effects (23). In the
clinical setting a few retrospective (19, 22) and prospective (17) observational
1

studies, investigating a total of about 1200 trauma patients, have found no difference
in adjusted mortality for hypothermic versus normothermic patients. However, other
retrospective (1, 3-8) as well as prospective (2) observational clinical studies,
investigating more than 750000 trauma patients have revealed that hypothermia
remains an independent determinant of mortality.
Accordingly, effective prehospital field protection and treatment of hypothermia,
is considered vitally important to improve the medical condition upon admission to
hospital, and active warming in the field is considered one important part of such
treatment (13-15, 26-32). Because the heat sources need to be portable and easily
handled by Search and Rescue (SaR) or Emergency Medical Services (EMS)
personnel, there are limited treatment options in the field and during prehospital
transport. Chemical heat pads, hot water bottles, carbon-fiber resistive heating
blankets, and charcoal fuelled heat pacs are commonly used and advised (13, 14, 26,
28, 29, 32). There are some laboratory studies (33-37) evaluating such fieldapplicable
devices (portable, not requiring external electrical power supply), but to
this author’s knowledge, only two randomized clinical trials have evaluated the
effectiveness of such heat sources in the field (38, 39).
As part of primary trauma care, it is important to have accurate measures to
evaluate the thermoregulatory state of the patient, both upon arrival of the rescue
personnel and during prehospital treatment and transport. In the field, especially in
harsh ambient conditions, this is often hard to achieve (14, 15). Measuring body
core temperature as well as skin temperature might be difficult and measuring
oxygen consumption to assess shivering is, in most clinical scenarios, not possible.
Thus, alternative measures, such as subjective judgement scales for assessment of
the thermal state of patients, might be of considerable importance in such scenarios,
both for an initial assessment of the patient and for evaluation of the treatment
provided. The judgement scales must be reliable and valid.
This thesis primarily focuses on protection and treatment of hypothermia in
prehospital trauma care, the emphasis being evaluation of active warming
intervention regarding impact on thermoregulation. Subjective judgement scales for
assessment of the thermal state of patients in a cold environment is also studied.
2

Throughout history, accidental hypothermia is perhaps best documented in military
history, where harsh ambient conditions have played a major role for the outcome of
numerous military campaigns. The list is long and distinguished.
Hannibal lost about 20000 of his 46000 soldiers in 218 B.C when crossing the
Pyrenees and Alps on his way towards Rome.
In 1718 Carl Gustaf Armfeldt and his army were caught in a blizzard crossing the
mountains on their return from Norway to Sweden. Only 1700 of the 5100 soldiers
survived.
In 1812 Napoleon lost much of his army because of the cold while attempting to
invade Moscow.
During the First World War 115000 British soldiers suffered local cold injuries or
trench foot.
During the Second World War 200000 German soldiers were disabled because of
cold injuries.
When writing about hypothermia from a historical perspective, it is also
important to mention the horrible crimes committed under the guise of medical
experiments on prisoners in German concentration camps during World War II. To
establish the most effective treatment for victims of immersion, Germans conducted
hypothermia experiments at the Dachau concentration camp in 1942 and 1943.
Immediately after, the American Chief of Counsel for War Crimes prepared a 228
page report after investigating the records of the experiments. This report, referred to
as the Alexander report (author: the American psychiatrist Leo Alexander), was
cited by some authors during the first decades after the war. This is considered by
most authors, including this author, strongly, ethically reprehensible. In addition,
citations are inappropriate on scientific grounds (40) [Berger 1990].
Traditionally, hypothermia has been defined as body core temperature < 35 ºC and
further classified into levels of severity based on the physiological changes that
occur because of decreased body core temperature (13-16). The levels are mild (32 –
35 ºC), moderate (28 – 32 ºC), severe (20 – 28 ºC), and profound hypothermia (< 20
ºC). These definitions were introduced to describe hypothermia resulting from
environmental exposure. However, more recently, due to the poor prognosis of the
combination of trauma and hypothermia, a revised classification has been developed
for use in trauma care (27). In this classification, hypothermia is defined as body
core temperature < 36 ºC and subsequently as 34 – 36 ºC for mild, 32 – 34 ºC for
moderate, and < 32 ºC for severe hypothermia. Furthermore, in this context, it is
3

important to stress that thermoregulatory responses are induced long before body
core temperature declines below the level defined as hypothermia. Beyond the two
classifications mentioned above, to be more applicable in a prehospital rescue
scenario, yet another method of staging hypothermic patients is recommended by
the International Commission for Alpine Rescue (ICAR) (28). Using this method,
degree of consciousness, presence or absence of shivering, cardiac activity, and
body core temperature are taken into consideration when deciding hypothermia
level. Table 1.1.
The human body maintains an average core temperature near 37 °C in various
thermal conditions (41). However, extreme thermal exposure as well as certain
medical conditions can lead rapidly to dangerous deterioration of body core
temperature. Because the speed of chemical reactions vary with temperature and
because the enzyme systems of the body have narrow temperature ranges in which
their function is optimal, normal body function depends on finetuned temperature
regulation. A deviation of about 2 °C above or below normal body core temperature
is well tolerated but a deviation of about 3 °C begins to threaten normal body
function.
Thermoregulatory mechanisms are not fully developed until after puberty and
from about seventy years of age thermoregulatory capacity is decreased, having the
consequence that children and elderly are more vulnerable to cold stress (41).
There are also differences between men and women (41). Women, as a group,
have, compared to men, a greater surface-area-to-mass ratio, a greater percentage of
subcutaneous and total body fat, a greater resting vasoconstriction in hands and feet,
a higher setpoint for cutaneous vasodilation and onset of sweating, and also cyclic
hormonal changes that influence thermoregulation. However, when individual
differences are accounted for, thermoregulatory gender differences are negligible.
4

The human body maintains core temperature by a fine balance between heat gain
and heat loss. When heat gain equals heat loss, a state of thermoneutrality exists, but
if heat loss exceeds heat gain, there is a risk of whole body cooling and hypothermia
(13, 14, 16, 41-43).
Factors governing heat exchange can be described by the heat balance equation:
Htot = ± Hd ± Hc ± Hr ± He
where
Htot = total metabolic heat production
Hd = conductive heat exchange
Hc = convective heat exchange
Hr = radiative heat exchange
He = evaporative heat exchange
Total metabolic heat production, which is about 70 W/m 2 (basal metabolism) in a
resting 70 kg man, increases with voluntary physical activity and involuntary muscle
contractions (shivering).
Conductive heat exchange means heat transfer by direct contact to a surrounding
medium, for example air, water, or solid ground (13, 41, 44, 45). The amount of heat
flow is dependent on physical characteristics of the surrounding medium,
temperature gradients, and the contact surface area. Air conducts heat poorly. Water
conducts heat approximately twenty five times, and metals up to ten thousand times,
greater than air.
Convective heat exchange means heat transfer by movements of the boundary
layer of the surrounding medium (air or water) (13, 41, 44, 45). The amount of heat
exchange is mainly dependent on the relative velocity of the surrounding medium.
Radiative heat exchange means heat transfer by infrared electromagnetic waves
to objects, for example, cold stone walls of a building (13, 41, 44, 45). The amount
of heat transfer is dependent on the physical characteristics of the objects and
temperature gradients.
Evaporative heat exchange means heat transfer from evaporation of water on the
skin surface or the respiratory tract, where the amount of heat exchange is dependent
on the amount of water evaporated (13, 41, 44, 45).
Heat loss is caused primarily by convection, which is greatly increased by wind
or movements (13, 41, 44, 45). To a smaller extent, heat is also lost through
radiation to cold objects in the surrounding environment, or to the clear sky, and by
5

evaporative and convective heat loss from the airways. Sweating and evaporative
heat loss from the skin is often minimal in cold environments, but could be
considerable in cases of wet clothing or skin due to immersion or previous physical
activity. In addition, if immersed, lying on the ground or in direct contact with a
cold surface, conductive heat loss will be significant.
Thermal receptors are widely distributed, especially in the skin, but also in the
abdominal viscera, the spinal cord, extrahypothalamic as well as hypothalamic
portions of the brain (13, 16, 41-43). The information from both peripheral and
central receptors is integrated in the preoptic nucleus – anterior hypothalamic area of
the brain, which is considered the center of thermoregulation. In response to cooling
of peripheral or central receptors or both peripheral and central receptors, a
sympathetic mediated thermoregulatory response is evoked, rendering
vasoconstriction to preserve heat within the body core and shivering to increase
endogenous metabolic heat production.
Vasoconstriction is primarily due to the closing of the arteriovenous anastomoses
of the hands and feet resulting in decreased blood flow in the entire extremity (13,
41, 42). During full vasoconstriction, blood flow through the fingers and toes can
decrease up to one hundredfold, from 80 -90 ml/min/100ml of tissue during full
vasodilation to 0,5 – 1,0 ml/min/100ml of tissue during massive vasoconstriction
(13). In addition to central regulation, tissue cooling directly affects vasoconstriction
by increasing cutaneous blood vessel sensitivity to cathecolamines.
In shivering, thermogenesis is due to involuntary and unsyncronized muscle
contractions (13, 41, 42). At maximum shivering, endogenous heat production can
be increased by as much as five times from resting levels (46). In mild hypothermia
(32 – 35 ºC), thermoregulatory responses are still intact, but if body core
temperature declines even further, thermoregulation, and thereby shivering, starts to
fail and at about 30 - 32 °C the shivering response is lost.
As a consequence of cold-induced peripheral vasoconstriction, temperature in
peripheral parts of the body starts to decline long before body core temperature is
affected (14, 41). Following substantial cold exposure, there is temperature
equalization between the warm body core and the cold peripheral parts, contributing
to a continuous fall in body core temperature, designated the afterdrop phenomenon.
Main contributing factors are conductive temperature equilibration between the
colder periphery and the warmer body core and circulatory changes involving
counter current cooling of warm blood circulating cold, previously vasoconstricted
peripheral tissues. The magnitude of the afterdrop, which can be considerable and
amount to several degrees, is dependent on temperature gradients in the tissues,
peripheral circulation, and endogenous heat production.
6

In trauma patients, thermoregulation is often impaired, resulting in an increased
vulnerability to cold exposure (13, 14, 16, 41). Injuries to the nervous system might
affect both vasoconstriction and shivering because of both central and peripheral
effects. Muscle injuries locally affect shivering ability. The administration of
analgesics, anaesthetics or both induces vasodilation and impedes shivering.
Although lipid and protein work as alternative fuels to carbohydrates in shivering
thermogenesis, malnutritive states reduce shivering capacity.
Psychological reactions to cold exposure is divided into thermal comfort or
discomfort and thermal sensation, where the fomer drives behaviour, while the latter
drives autonomic thermoregulation, described above (47). However, in clinical
reality it is difficult to differentiate between those modalities. Because of thermal
comfort is driven by both physiological and psychological variables, whereas
thermal sensation is driven by only physiological variables, in this thesis,
psychological reactions to cold exposure is described as thermal comfort.
According to the American Society of Heating, Refrigerating, and Air-
Conditioning Engineers (ASHRAE), thermal comfort is described as a state of mind
that expresses satisfaction with the surrounding environment. Physiological
variables affecting thermal comfort include information from brain, body core and
skin temperature sensors as well as vasoconstriction and shivering (48-52). The
psychological variables include previous experience with cold exposure, state of
mind, social context, and behaviour (53).
7

The following section outlines, the pathophysiological reactions and clinical picture
due to cold stress and hypothermia. In this context, attentiveness to the occurrence
of considerable individual differences is important. There are also differences
dependent on whether hypothermia has developed rapidly (acute hypothermia) or
over a longer period (chronic hypothermia) (14).
Respiratory and cardiovasculatory changes. In response to the cold-induced
sympathetic mediated thermoregulatory stress response, respiratory and cardiac
work is increased. Shivering thermogenesis increases oxygen demand and peripheral
vasoconstriction raises systemic blood pressure which further escalates cardiac
workload. (13, 16, 41, 42, 46).
As body core temperature declines, spontaneous depolarization of the pacemaker
cells of the sinus node decreases, resulting in atropine-resistant bradycardia (16).
Because hypothermia increases duration of action potentials, and because the His-
Purkinje system of the heart is more sensitive to cold than the myocardium itself,
decelerated conduction might cause re-entry currents, resulting in ventricular
arrhythmias. PR, QRS, and QT intervals are prolonged (54-56). However, not
pathognomonic of hypothermia, Osborn or J-waves, a distinctive deflection occuring
at the QRS-ST junction, are common in hypothermic patients. Osborn waves might
pose a diagnostic difficulty, because they, when pronounced, resemble the elevation
of the ST segment also seen in transmural myocardial infarction. Atrial as well as
ventricular arrhythmias are encountered at body core temperatures below 32 °C and
spontaneous ventricular fibrillation is seen below 25 °C (15). The risk of developing
cardiac arrhythmias is increased by hypovolemia, tissue hypoxia, electrolyte and
acid balance disturbances, as well as rough handling of the hypothermic patient.
In chronic hypothermia, peripheral vasoconstriction, resulting in increased central
blood volume, will lead to fluid extravasation and tissue oedema, including lung
oedema and also to increased diuresis due to raised renal perfusion pressure (14).
Thus, patients exposed to prolonged cold stress should be considered hypovolemic,
which is important to account for when peripheral circulation returns to normal
during rewarming.
As for the heart rate, respiratory rate also declines with body core tempreature. In
severely hypothermic patients, breathing may be hard to detect since it is very slow
and shallow.
Neurological changes. The cold-induced sympathetic mediated stress response
results in an initial increase in cerebral metabolism, but as body core temperature
drops by more than 1 °C, cerebral metabolism will decline (15). This will result in
an increasingly deteriorated mental status, presented as impaired memory and
judgement, slurred speech, and decreased level of consciousness. Most patients are
unconscious at a body core temperature below 30 °C (16).
8

Muscle performance in the extremities is impaired both because of local tissue
cooling as a consequence of vasoconstriction and neurological malfunction. These
combined effects pose a substantial risk for development of local cold injuries (57,
58).
Blood chemistry changes. Haematocrit and also blood viscosity rises because of
fluid extravasation and increased diuresis (15, 16). Platelet count is lowered due to
bone marrow depression and sequestration in the spleen and liver. Platelet adhesion
and aggregation is impaired. Coagulation enzyme activity is reduced to a level
equivalent to 50 % of clotting factor deficiency at temperatures below 33 °C (59,
60). Already mild hypothermia induces a general coagulopathy (61, 62) which can
be reversed by active warming (63).
Leukopenia, also due to bone marrow depression and sequestrian of leukocytes in
the spleen and liver, together with a general immune system depression, weakens the
resistance to infections(16).
Serum electrolytes fluctuate over time and with body core temperature during
cooling and rewarming (14, 16). Hypokalaemia due to an intracellular redistribution
of potassium, as well as hyperkalaemia, due to assumed cellular membrane
dysfunction, is seen. Hyponatremia is common in chronic hypothermia due to
osmolar diuresis.
Neutral pH varies with temperature and rises with body core cooling (14).
Respiratory alkalosis due to initial hyperventilation is common after sudden
immersion in cold water. This is followed by respiratory and metabolic acidosis due
to respiratory depression and also ketogenesis and lactate formation from shivering,
reduced cardiac output and tissue hypoxia due to impaired peripheral circulation.
Initially, blood glucose level rises because of glycogenolysis caused by the coldinduced
stress response (14, 16). During prolonged cold stress glycogen stores will
be depleted and hypoglycaemia might develop due to shivering and the cold-induced
glycosuria.
Actions to reduce cold stress and prevent further heat loss are an important and
integrated part of prehospital trauma care. Initial measures should be taken to
shelter, remove wet clothing, and insulate the patient from ambient weather
conditions and ground chill. Adequate windproof and waterproof insulation
ensembles (passive warming) are imperative. In addition, depending on the
physiological status of the patient including body core temperature, available
resources and expected duration of evacuation, the application of heat (active
warming) is recommended by most authors as protection from further cooling
during treatment and transport to definitive care (13-15, 28-32). In a prehospital
setting the primary objective of active warming is to reduce cold stress and further
9

cooling, not to rewarm the patient. Available, field applicable, heat sources all have
limited heating capacity, rendering rapid rewarming in the field impossible. Too
rapid rewarming, implying possible development of fluid, acid-base or electrolyte
imbalances that would be difficult to control in the field, is therefore not considered
a problem (14, 15, 29).
Several studies on mildly hypothermic shivering subjects have found that
exogenous skin heating attenuates shivering heat production by an amount
equivalent to the heat donated (34, 37, 64). Thus, in a mildly hypothermic, shivering
patient, active external rewarming generally does not decrease afterdrop or increase
the rewarming rate more than shivering thermogenesis. However, thermogenesis
from shivering during passive warming alone can result in significant anaerobic
metabolism and lactic acidosis, and active external warming might therefore present
treatment advantages of decreased respiratory and cardiac workload, preserved
substrate availability, and increased comfort. Overall, additional active warming
should be considered also when treating mildly hypothermic trauma patients.
When shivering is diminished or absent, as in moderate to severe hypothermia, or
is otherwise impaired because of the overall medical condition of the patient, active
external or internal warming is required. Otherwise afterdrop will continue and little
or no warming will occur (35, 36, 65, 66).
To be field applicable, warming modalities need to be portable and require no
external electrical power supply. In a report summarizing survey responses from
from 41 Mountain Rescue Association teams (67) the active warming methods most
frequently used were warm IV fluids, chemical heat pads, body-to-body warming,
charcoal heaters, and warm air or oxygen inhalation. Some of these methods are
extensively studied, others are not. Regardless of the warming method, most of the
studies are laboratory studies, only a few have been conducted in a prehospital
environment, which might be a considerable limitation when making decisions
about prehospital trauma care.
In the following section, field applicable warming methods, i.e. warming methods
that are portable and require no external electrical power supply, are described.
Exercise. Moderately to severly hypothermic patients are likely to be physically
and metabolically exhausted and have an altered level of consciousness. In these
patients, movements might also induce ventricular fibrillation (13-16). Thus,
excercise is not a treatment alternative. However, in mild hypothermia, increasing
endogenous heat production by exercise has been suggested for warming purposes.
In one study on subjects cooled to 33 °C, Giesbrecht et al. (34) found that the post
cooling rewarming rate for exercise was significantly higher than for shivering, but
not higher than for external heat. However, exercise, as well as external heat,
10

significantly increased both length and amount of afterdrop compared to shivering,
resulting in no difference between the three treatment alternatives regarding total
recovery time. In another study on shivering subjects, Giesbrecht et al (68)
postponed exercise until afterdrop was complete and found that during exercise
there was a second afterdrop of similiar, but not greater, magnitude as during the
initial shivering period.
Body-to-body warming. Direct body-to-body contact with a minimally clothed
euthermic heat donor used to be widely recommended for warming hypothermic
patients in the field (69-71). As it is also shown for other active external heat
sources (34, 37), body-to-body warming has no beneficial effect on body core
warming compared to shivering thermogenesis alone (64, 72). However, if shivering
is diminished or absent, as in moderate to severe hypothermia, or otherwise
impaired because of the overall medical condition of the patient, heat donated by
body-to-body contact will, as other external heat sources, increase heat gain (36).
Heat packs. Warm water bottles, chemical heat pads, or the HeatPac®; a
charcoal heater (Normeca AS, Oslo, Norway), are widely used (67) for active
external warming in the field. Such devices are portable and easy to handle for the
rescue personnel, but heat content is limited. In order facilitate heat transfer, heat
sources should be applied in proximity (precautions should be taken to avoid burn
injuries) to the skin on areas with high heat transfer such as the chest (36), neck,
axillae, and groins.
In mildly hypothermic shivering subjects, laboratory studies of the Heat Pac®
(34, 37) have shown no beneficial effect on body core rewarming compared to
shivering thermogenesis alone. This is in accordance with studies on other active
external warming modalities (64, 72). In one laboratory study on human subjects,
where shivering was suppressed pharmacologically to resemble moderate to severe
hypothermia, the HeatPac® was significantly more efficient than body-to-body
rewarming in minimizing afterdrop and facilitating body core rewarming (36). The
HeatPac® consists of a combustion chamber, charcoal fuel, and a branched;
reinforced, but flexible, heating duct and produces 250 W of heat. It is placed on the
patient’s chest and the heating ducts are applied dorsally over the shoulders and then
anteriorly under the axillae crossing over the lower chest. Total skin contact surface
area of the chamber (23 × 12 × 6 cm, 1100 g) and ducts is about 1500 cm2. Because
there are no clinical studies using the HeatPac®, practical aspects, such as ignition
of the charcoal fuel in field conditions and the potential risk of the charcoal fuel
burning in proximity to or even inside vehicles, has not been evaluated. However, a
safety concern regarding carbon monoxide contamination has been presented (37).
In a prehospital clinical randomized trial by Watts et al (39), chemical heat pads
were applied in at least two locations (on top of the head, under the patient against
the lower back, or under either axilla with the hot pack against the chest wall) This
treatment was compared to five other treatment alternatives (no intervention, passive
rewarming, reflective blankets, warmed IV fluids, and warmed IV fluid plus
11

eflective blanket). Trauma patients who received hotpack warming showed a mean
increase in body core temperature during transport (0.7 °C) (measure of variation
not reported), while all other groups (no intervention, passive warming, reflective
blankets, warmed IV fluids, warmed IV fluid plus reflective blanket) showed a mean
decrease in temperature during transport (- 0.2 °C to - 0.4 °C), the difference was
statistically significant. The chemical heat pad Hot Cycle 1 (Sign Manufacturing
Corporation, Fairfield CA) is activated by breaking an internal chamber and it
reaching a temperature of 54.5 °C. To avoid burn injuries of the skin the chemical
heat pads were rolled in towels before application.
Resistive heating. In a laboratory study on human subjects by Greif et al (35),
where shivering was suppressed pharmacologically to resemble moderate to severe
hypothermia, a carbon-fiber resistive heating blanket was evaluated. Both total body
heat content and body core rewarming rate were increased compared to passive
warming alone. In contrast to the passive warming group that presented a small,
although not statistically significant afterdrop, there was no afterdrop in the carbonfiber
resistive heating blanket group. The carbon-fiber resistive heating blanket
measures 80 x 200 cm, with the actively heated section being 40 x 148 cm. The
batteries weigh 0.5 kg, each lasting 30 – 40 minutes.
In a prehospital clinical randomized trial by Kober et al (38)[Kober 2001] the
same carbon-fiber resistive heating blanket was compared to passive warming alone.
In trauma patients receiving passive warming alone mean body core temperature
decreased by 0.4 °C/h (95% CI; 0.3 – 0.5 °C/h), whereas in patients receiving
additional carbon-fiber resistive heating, mean body core temperature increased by
0.8 °C/h (95% CI; 0.7 – 0.9 °C/h), the difference between groups being statistically
significant.
Inhalation warming. The effectiveness of inhalation rewarming has been
somewhat equivocal. In some studies (73-76), beneficial effects on body core
temperature have been reported, whereas in other (37, 77, 78), including one study
on non-shivering human subjects (66), no body core warming advantages have been
found. Benefits of inhalation warming such as rehydration, stimulation of
mucociliary activity in the respiratory tract and direct heat transfer from the upper
airways to the hypothalamus, brain stem, and other brain structures have been
suggested (75).
Warm intravenous fluids. Intravenous fluids should be heated to 40 - 42 °C
during hypothermia resuscitations (32). Cold fluid resuscitation of hypovolemic
patients can induce hypothermia, infusion warming devices are therefore mandatory
during massive volume resuscitations (79, 80).
12

To have accurate measures to evaluate the thermal state of patients in the prehospital
setting is vitally important. In the field, especially in harsh ambient conditions, this
is often hard to achieve (41). Adequate measurements of body core temperature as
well as skin temperature might be difficult to obtain (14, 41), and measuring oxygen
consumption for assessment of shivering is, in most rescue scenarios, not possible.
Thus, alternative measures such as subjective judgement scales for assessment of
the patient’s thermal state might be of considerable importance, both for an initial
assessment of the patient and for evaluation of the treatment provided.
The temperature of the pulmonary artery is considered the best reference
temperature for deep body core temperature (41). However, pulmonary artery
catheterization is not an option in the field.
In the following section, non-invasive generally accepted sites for measuring
body core temperature are listed, all of various degrees of usefulness in the field.
Oesophagus. Oesophageal temperature is obtained by placing a temperature
probe into the distal portion of the oesophagus, usually via nasal passage, to the
level of the heart (81). The proximity of the oesophagus to the descending aorta and
the left auricle is the anatomical basis for an accurate measurement of deep body
core temperature. It is an accurate method for deep body core temperature (82, 83).
However, this method is, for educational and practical reasons, also not an option in
many prehospital rescue scenarios.
Closed ear canal. Closed ear canal temperature is obtained by placing a
temperature probe in the mid to distal portion of the ear canal, which is then sealed
from the ambient environment by insulation covering the ear. The proximity of the
ear canal to the internal carotid artery makes it an ideal site for measuring body core
temperature. This method has been shown to correlate well with oesophageal
temperature (84, 85). Although these results are mainly based on data from indoor
operating theatre environments, Walpoth et al. (84) have shown that, if properly
sealed from the ambient air, closed ear canal temperature is also reliable in sub-zero
and wind conditions. As the reliability of closed ear canal temperature is dependent
on positioning within the ear canal, there is a risk of false low recordings. However,
if measurements are made over a period of time with the probe left in position in the
ear canal, which is standard procedure, temperature change over that period of time
should be reliable.
Tympanic membrane. Tympanic membrane temperature is obtained by
reflecting infrared electromagnetic waves from the tympanic membrane and the ear
canal. It is a simple field applicable method but has been shown not reliable (86-88).
13

Rectum. Rectal temperature is an accurate method for measuring deep body core
temperature in steady state conditions (41). However, when deep body core
temperature changes, there is a delay in rectal temperature change.
Urinary bladder. Urinary bladder temperature is an accurate method for
measuring deep body core temperature and as many patients need a urinary catheter,
the catheter can be used for measuring temperature (89).
Oral cavity. Oral temperature is obtained by placing the probe in the sublingual
pocket which is well perfused. Provided that the patients can keep their mouths
closed, it is a simple field applicable method, but has also been shown not reliable
(88).
The most common single item judgement scales are Visual Analouge Scales (VAS),
Numerical Rating Scales (NRS) and Verbal Rating Scales (VRS). A VAS consists
of a visual line, usually 100 mm long, where the ends of that line are labeled with
descriptions for the extremes of the studied modality. The respondent places a mark
on the line representing his or her level of experienced intensity in relation to the
described extremes. Instead of a visual line, a NRS consists of a range of numbers,
usually 0 – 10, and a VRS consists of a list of words or phrases, describing various
degrees of the studied modality.
The international standard BS EN ISO 10551:2001 (90) outlays general
principles for construction of subjective judgement scales for assessment of the
influence of the environment on the thermal state of the patient. These general
principles, mainly used for indoor or near isothermal environments, recommend
symmetrical 7 to 9-degree rating scales comprising a central indifference point and
two times 3 or 4 degrees of increasing intensity for both hot and cold. The two most
well-known scales are the Bedford scale (91) and the ASHRAE scale (92). In 1936
Bedford (91) collected data on almost 2000 industrial workers to correlate subjective
judgements of their thermal comfort to objective measurements of the thermal
environment. The responses of the workers were measured according to a seven
degree VRS, called the Bedford scale, and to be able to use statistics on the data,
numerical values were assigned to the different levels of the scale. In 1971 Rohles et
al. (92) collected data on 1600 college students to correlate subjective judgement of
thermal comfort to ambient air temperature, humidity, length of exposure and
gender. The scale developed for those studies is also a VRS, called the ASHRAE
scale.
14

The overall objective of this thesis was to evaluate prehospital active external
warming intervention regarding impact on thermophysiological and psycological
reactions. Reliability and validity of the Cold Discomfort Scale (CDS), a subjective
judgement scale for assessment of the thermal state of patients in a cold environment
was also studied.
Specific objectives were to evaluate:
the warming effectiveness of three different portable heat sources, none of them
requiring external electrical power supply, and spontaneous warming (control
condition) regarding their impact on post-cooling body core temperature including
afterdrop in a laboratory setting (study I).
external warming intervention, using a previously evaluated (study I) heat source
as additional treatment to a standard protocol of passive warming in a prehospital
clinical setting, both during transport and treatment in a heated road ambulance or
helicopter (study II) and during outdoor field treatment and transport (study III),
regarding impact on body core temperature (study II) and thermal comfort (study II
and III).
the CDS, a subjective judgement scale for assessment of the thermal state of
patients in a cold environment, regarding reliability, defined as test – retest stability,
and criterion validity, defined as ability to detect changes in cumulative cold stress
(study IV)
.
15

Because of small study populations, especially in the laboratory studies, where study
populations also are relatively homogenous, the limited external validity must be
considerd when conclusions are drawn based on the results of these studies.
16

The CDS was evaluated in a laboratory environment regarding reliability, defined as
test-retest stability and criterion validity, which is defined as the ability to detect
changes in cold discomfort due to changes in cumulative cold stress (study IV).
Twentytwo healthy subjects participated in two trials each, on two separate
occasions, at about the same time of day, approximately one week apart. Conditions
were identical during both trials, subjects were exposed to -20 °C for 60 minutes
wearing only light clothing. CDS ratings were recorded every five minutes, both
during 10 minutes of base line data collection and during the 60 minutes of cold
exposure. The CDS is a subjective judgement scale for assessment of the thermal
state of patients in a cold environment. It is designed as a NRS, where the subjects
assess the thermal state of their whole body, not specific body parts, and provide
integer values from 0 to 10, where 0 means not being cold at all and 10 means being
unbearably cold.
Pre-study calculations indicated a minimal sample size of 18 to detect a difference in
Cold Discomfort Scale ratings of 2 or more IQR; 2) presupposed 80% statistical
power at an -level of 0.05. Reliability of the CDS was analysed for test-retest
stability using weighted kappa coefficient, comparing median CDS ratings between
the two trials. This included all the measurements made every five minutes and also,
separately, every single measurement.
Criterion validity was analysed by comparing median CDS ratings over a moving
30 minutes interval (5-35 minutes; 10-40 minutes; 15-45 minutes etc) using
Wilcoxon signed ranks test. Statistical significance was defined as p < 0.05, and, in
analysis of criterion validity, after correction for multiple comparisons according to
Bonferroni as p < 0.008 (two-sided).
Because the test and retest were conducted in a laboratory environment, ambient
conditions were controlled and identical for both the test and retest. Subjects also
served as their own controls. However, it is always difficult to achieve identical
conditions in a test-retest design when measuring subjective parameters. Even
though all arrangements are made to match, the subjects might react differently to
the same cold exposure at two different occasions. There might also be a habituation
which can either increase or decrease the sensitivity to the exposure.
Evaluation of criterion validity is dependent on how it is defined and conclusions
about criterion validity therefore, must be based on that definition. It might
17

sometimes be hard to appraise if the chosen definition is relevant, which then
complicates interpretation of results.
In this study, criterion validity was defined as the ability to detect changes in cold
discomfort due to changes in cumulative cold stress by subjects exposed to -20 °C
wearing only light clothing, over a moving 30-minute interval. The time interval
was chosen based upon what, under prevailing conditions, was appraised a clinically
significant change in cumulative cold stress. A moving 30-minute interval was
chosen in attempt to cover early as well as late changes within the evaluation period
of 60 minutes.
In a laboratory environment, three different portable exogenous heat sources, none
of them requiring external electrical power supply, and spontaneous warming
(control condition) were compared (study I). Five human subjects participated in
four trials each, one trial for every condition, thereby serving as their own controls.
To mimic moderate to severe hypothermia and also to achieve equal conditions of
endogenous heat production for each condition, shivering was suppressed
pharmacologically using a human model previously evaluated in several studies (36,
65, 66). An initial cooling phase, in which subjects were exposed to 8 º C water for
10-30 minutes, was followed by a 120-minutes treatment phase including passive
warming with the addition of either one of three exogenous heat sources or
spontaneous warming. Prior to the trials, 30 mg of orally administered buspirone,
and during the last 10 minutes of the cooling phase, and also if needed during
rewarming, intravenously administered meperidine to a maximum cumulative dose
of 3,5 mg/kg, was used to supress shivering. Oesophageal temperature (81-83), skin
heat transfer, skin temperature, oxygen consumption, respiratory rate and heart rate
were continuously monitored and recorded during the trials. The exogenous heat
sources applied to the chest and upper back, were a charcoal heater, HeatPac®,
(Normeca AS, Oslo, Norway), two chemical heating pads (Dorcas AB, Skattkarr,
Sweden) and two flexible water bags (Mountain Safety Research, Seattle, WA) each
filled with 2 litres of 55 °C water replenished every 20 minutes.
Continuous data, considered normally distributed, were initially compared using
repeated measures analysis of variance (ANOVA). If statistical significance was
revealed, Student´s t-test for pair-wise post hoc analysis with Fisher’s protected least
significant difference (PLSD) for multiple comparisons test was used to identify and
quantify differences between individual conditions. Statistical significance was
defined as p < 0.05 (two-sided).
18

The human model, in which shivering is suppressed pharmacologically, is designed
to thermophysiologically mimic a scenario of moderate to severe hypothermia. It has
been evaluated in several studies (36, 65, 66). Beyond the ability to evaluate
different treatment alternatives in a thermophysiologically hypothermic subject,
which, for safety reasons, would otherwise be impossible, this model also makes
comparisons between treatment alternatives almost perfectly fair regarding influence
of endogenous heat production. This is because endogenous heat production is kept
at a constantly low rate and also is controlled for. If there are differences in
endogenous heat production, this will be revealed and impact on the overall result
will be possible to analyse.
The human model for severe hypothermia eliminates and controls for shivering.
However, it does not account for other physiological or psychological changes that
come with moderate to severe hypothermia such as, respiratory, circulatory,
neurological, and blood chemistry changes.
In this study, because of safety considerations, the amount of meperidine
administered to inhibit shivering was limited to a cumulative dose of 3.5 mg/kg for
each trial. Therefore, the cooling phase had to be adjusted for the subjects’ body
composition, resulting in a relatively longer cooling phase in subjects with a higher
percentage of body fat. However, because subjects served as their own control and
the cooling phase was identical for each subject for all conditions, this should have
very limited impact on the results.
Parametric statistical tests were used for ratio and interval scale data, despite
small study population, because data were considered normally distributed.
However, considering the small study population, the use of parametric statistics is
controversial and therefore, results are also presented for non-parametric statistics
using Friedman test and Wilcoxon signed rank test instead of ANOVA and PLSD
respectively.
In two randomized clinical trials, chemical heat pads (Dorcas AB, Skattkarr,
Sweden), the same chemical heat pads as previously evaluated on non-shivering
subjects (study I) were used to evaluate the impact of active warming on
thermoregulation in mildly hypothermic trauma patients with preserved shivering
capacity during transport and treatment in a heated road ambulance or helicopter
(study II) or during outdoor field treatment and transport by ski patrol units (study
III). Sequential trauma patients, age 18 years, who had sustained an outdoor
injury, were enrolled. Patients were excluded if initial level of consciousness was
affected, (Glasgow Coma Scale < 15), if duration of transport or treatment was
expected to be shorter than 10 minutes, if active warming already had been initiated,
19

if they had been taken indoors for more than 10 minutes before ambulance or ski
patrol unit arrival or had an initial cold discomfort rating 2.
If enrolled, patients were randomized to either passive warming with blankets
according to an existing protocol alone (study II: n = 22, study III n = 9) or to
passive warming with blankets according to the existing protocol with the addition
of one chemical heat pad applied on the anterior chest (study II: n = 26: study III: n
= 11). In study II, after loading into the ambulance or helicopter, initial
measurements of closed ear canal temperature (84, 85), respiratory rate, heart rate,
blood pressure, and patients’ subjective sensation of thermal comfort according to
the CDS were made. In study III, at the scene of injury, an initial measurement of
patients’ subjective sensation of thermal comfort according to the CDS was made.
In both studies initial measurements were made before eventual application of the
chemical heat pad. Measurements were then continuously made every 30 minutes
and at the receiving hospital (study II), first aid centre or EMS unit arrival (study
III).
Pre-study sample size calculations, estimating a difference in body core temperature
of 0.5 °C and in CDS ratings of 2 between the two intervention groups, an
alpha of 0.05 and a beta of 0.10 revealed a minimum study population of 42
patients. This was for both studies II and III since CDS was the restricting variable.
For comparison between groups the Mann Whitney U test was used for interval
and ordinal data and Chi square or Fisher’s exact test for nominal data. In addition,
CDS ratings were characterized as increased, unchanged or decreased and the
difference between groups was analysed using Fisher’s exact test. For comparisons
within groups, Wilcoxon signed rank test was used for interval and ordinal data.
Statistical significance was defined as p < 0.05 (two-sided).
The rescue personnel used the standard protocol for all patients; participation in the
studies involved no interfering instructions. Application of the chemical heat pad to
the anterior thorax for those assigned to additional active warming was the only
difference between groups. This study design thus enables a fair comparison
between study groups. The active warming intervention being evaluated in a proper
environment must also be considered a great strength of the study. A laboratory
environment results in a high degree of control when conducting the study,
including control of evaluated intervention and recordings. This might be more
difficult to achieve in a prehospital clinical setting and it is important to be aware of
such possible sources of errors when interpreting results. Although, if considered,
that must not entail a limitation.
Closed ear canal temperature (84, 85) was used for measurement of body core
temperature. Because the absolute value of closed ear canal temperature is
somewhat dependent on placement within the ear canal, those values might not be
20

completely accurate. However, if the ear canal is properly insulated from the
ambient environment and if the temperature sensor is kept in the same place during
the entire trial, recordings of temperature changes will be accurate.
CDS was used for assessment of thermal comfort. The reliability (defined as test
– retest stability) and criterion validity (defined as ability to detect an increase in
cumulative cold stress) of CDS is discussed below.
In study II, fourteen day and night manned road ambulance units and one
helicopter ambulance unit participated during the entire, or part, of an inclusion
period of two and a half years. There are 125 000 inhabitants in the catchment area
of the participating ambulance units and, due to tourism, the population increases
during winter time. In study III, ski patrols from three ski resorts in the northern
parts of Sweden participated in the study during two consecutive winter seasons.
There were a total number of 1 803 000, person-ski days during that period. Still, the
study population was relatively small, which might be due to several factors. The
compliance of the rescue personnel to include patients in the study might have been
low. The doctors in charge of the studies, Lundgren and Henriksson, visited every
unit about twice a year and above that carried out monthly phone calls to make sure
the study proceeded as planned, but that might not have been adequate.
The inclusion and exclusion criteria were also somewhat narrow, which also
might have had an influence on the size of the study population. Reports from the
ambulance personnel indicated that many of the patients injured outside had already
been moved inside, while waiting for the ambulance to arrive and, due to long
distances, the time limit of ten minutes of indoor stay or active warming treatment
was exceeded. Reports from ski patrol units indicated that many of the patients
injured and in need of treatment on the ski slopes were rapidly transported to an
indoor medical facility, thereby outdoor treatment and transport time were less than
the required 10 minutes.
Patients having an initial CDS rating of 2 were also excluded since they were
not considered cold stressed.
However, presented inclusion and exclusion criteria were considered necessary to
limit the study population to those actually considered cold stressed, even if some
patients who also could have been considered cold stressed might have been left out,
and the resulting consequenses was a small study population.
Study II was terminated when the required number of included patients had been
reached according to prestudy sample size calculation. Statistical analysis of
outcome variables was performed until the second measurement, at an average of 26
± 7 minutes, since at that point, all 48 subjects were included, whereas at the third
measurement, performed at an average of 58 ± 5 minutes only 12 subjects remained.
Study III was terminated before the required number of included patients had
been reached according to pre-study sample size calculations, since differences in
CDS between groups proved to be larger than expected when making those
calculations.
21

CDS data was considered ordinal scale data. In study II, non-parametric statistics
wass used also for interval scale data as the study population was small.
22

In total, forty-four trials were completed; all twenty-two subjects participated in two
trials each. Median CDS increased from 0 (IQR; 0 – 0) during base line to 7 (IQR; 5
– 7) at the end of the first trial (test) and to 6 (IQR; 5 – 7) during the second trial
(retest) (Figure 2).
10
9
8
7
6
5
4
3
2
1
0
0 10 20 30 40 50 60
23

Reliability analysed for test-retest stability, using weighted kappa coefficient, which
included all the measurements made every five minutes, was 0.84 and separated for
every single measurement between 0.48 and 0.86 (Table 3).
24

Criterion validity analysed by comparing median CDS ratings (n = 22) over a
moving 30-minute interval, revealed that CDS ratings were significantly increased
during each 30-minute interval (5-35 minutes; 10-40 minutes;15-45 minutes etc)
(Table 4).
25

Metabolic heat production increased from 116 ± 17 W (mean ± SD) during baseline
to 195 ± 51 W during the last 10 minutes before meperidine injection. Meperidine
suppressed shivering and heat production returned to 114 ± 21 W during the first 40
minutes of treatment and subsequently fell to 97 ± 17 W throughout the remaining
80 minutes of treatment (Figure 3). There were no significant differences in
metabolic heat production for the different conditions either during the cooling or
the treatment phase.
300
250
200
150
100
50
0
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120
26

Parametric statistics: The post-cooling afterdrop was significantly less for the
chemical heat pads and the hot water bags, but not for the charcoal heater, compared
to spontaneous warming. Time to temperature nadir and temperature drop during the
entire treatment phase (0 to 120 minutes) was significantly less for all three heat
sources compared to spontaneous warming.
Non parametric statistics: The postcooling afterdrop was significantly less for the
chemical heat pads, but not for the hot water bags or the charcoal heater, compared
to spontaneous warming. Time to temperature nadir and temperature drop during the
entire treatment phase (0 to 120 minutes) was significantly less for all three heat
sources compared to spontaneous warming. (Table 5)
27

Change in core temperature (°C)
Change in core temperature (°C)
1,0
0,0
-1,0
-2,0
-3,0
1,0
0,0
-1,0
-2,0
-40 -20 0 20 40
Time (min)
60 80 100 120
Change in core temperature (°C)
Change in core temperature (°C)
1,0
0,0
-1,0
-2,0
-3,0
0,0
-1,0
-2,0
-40 -20 0 20 40
Time (min)
60 80 100 120
1,0
-3,0
-40 -20 0 20 40 60 80 100 120
Time (min)
28
-3,0
-40 -20 0 20 40
Time (min)
60 80 100 120

The mean time from injury to the arrival of ambulance personnel arrival was 64 (95
% CI; 41 – 88) minutes in patients assigned to passive warming only and 81 (95 %
CI; 61 – 101) minutes in patients assigned to additional active warming with no
significant differences between the two groups.
During road and air ambulance transport ear canal temperature was significantly
increased and CDS ratings was significantly decreased, both in patients assigned to
passive warming only, and in patients assigned to additional active warming (Table
6).
However, when change in cold discomfort was characterized as increased,
unchanged or decreased, 15 out of 21 in the group assigned to passive warming only
presented a decrease in CDS rating, whereas all 26 patients in the group assigned to
additional active warming presented a decrease in CDS ratings. This difference in
CDS rating change between groups was statistically significant (p < 0.05).
The time from injury to ski patrol arrival was 11 ± 7 minutes (mean ± SD) in
patients assigned to passive warming only and 16 ± 9 minutes in patients assigned to
additional active warming, with no significant differences between the two groups.
During field treatment, CDS ratings were significantly reduced in patients
assigned to additional active warming, but remained the same in patients assigned to
passive warming only (Table 7).
29

In addition, when change in cold discomfort was characterized as increased,
unchanged, or decreased, 9 of 11 patients assigned to additional active warming
presented a decrease in cold discomfort and two remained at their initial cold
discomfort whereas only 3 of 9 patients assigned to passive warming alone
presented a decrease in cold discomfort; one remained at initial cold discomfort and
5 presented an increase in cold discomfort, this difference in cold discomfort change
between groups being statistically significant (p < 0.05).
30

Admission hypothermia is an independent risk factor associated with worse outcome
in trauma patients, retrospective (1, 3-8) as well as prospective (2) observational
clinical studies have revealed that hypothermia remains an independent determinant
of mortality after correction for severity of injury. Actions to reduce cold exposure
and prevent further heat loss are therefore an important and integrated part of
prehospital trauma care. Active warming is by most authors recommended to aid in
protection from further cooling during treatment and transport to definitive care (13-
15, 28-32). Prehospital active external warming treatment, aims primarily at
reducing further heat loss and to counteract the post cooling afterdrop.
Chemical heat pads and warm water bottles are commonly used and advised for
prehospital active warming treatment. However, unlike the HeatPack® charcoal
heater, these heat sources have never before been evaluated regarding their impact
on thermoregulation in non-shivering hypothermic subjects. Although all the
evaluated heat sources were applied on the chest and upper back, providing their
heat content conductively to the skin and underlying tissues, they displayed some
important differences that affected warming effectiveness.
Chemical heat pads were most effective in attenuating afterdrop amount. This
was most likely due to their high initial heat delivery to a relatively large surface
area. The charcoal heater, also with a high initial heat production but a relatively
small surface area, had less effect on afterdrop amount compared to spontaneous
warming than the chemical heat pads.
The charcoal heater, like the hot water bags (replenished every 20 minutes),
provided high continuous heat delivery. The heat delievery of the chemical heat
pads declined over time. Therefore, the charcoal heater also, like the chemical heat
pads and the warm water bottles, presented a significant difference in body core
temperature change over the entire warming phase, compared to spontaneous
warming. Thus, all heat sources presented a statistical significant lower body core
temperature drop compared to spontaneous warming during the warming phase and
this is also in accordance with findings of large afterdrop amounts and little or no
warming, when exogenous heat is left out (35, 36, 65, 66).
In summary, heat sources applied on the chest and upper back were effective to
attenuate afterdrop when providing high heat content over a large surface area, and
31

effective to continue to increase body core temperature when providing sustained
high heat content.
However, it could be discussed whether recorded differences in body core
temperature of about 1˚C is clinically relevant. Nevertheless, prehospital active
warming aims primarily at reducing heat loss and to reverse a continuing fall in
body core temperature. Thus, especially at critical temperature levels at about 30 ˚C
where the heart becomes susceptible to arrhythmias, every degree might be
important or even lifesaving.
All of the evaluated warming modalities are portable, require no external power
supply and are easily used by laypersons, Search and Rescue (SaR) personnel, or
Emergency Medical Services (EMS) crew members without significant training.
They are suitable for different clinical scenarios.
The charcoal heater, which is lightweight and provides heat over 8–12 hours, has
advantages in protracted evacuation and rescue operations because of sustained heat
production.
Water bags, which often are lightweight and easy to bring during backcountry
excursions are better suited for scenarios in which the patient will remain on the
scene of the accident waiting for evacuation, because an external heat source and
significant effort are required for replenishing the water bags.
Chemical heating pads, although heavier and requiring more space than the other
modalities can be easily transported in a vehicle such as in ground or air ambulance
units. As they are effective in reversing the initial fall in body core temperature, they
might prove valuable for initial thermal stabilization of a cold patient, and if
replaced at sufficient intervals, also for continuous heat delivery.
Passive warming according to standard treatment protocol was effective in
preventing afterdrop and slowly increasing body core temperature during transport
to definitive care (study II). Additional active warming composed no beneficial
effect on body core temperature in this study on cold stressed trauma patients with
an initial body core temperature of about 35°C and preserved shivering capacity.
This is in accordance with previous laboratory studies on mildly hypothermic
shivering subjects, where exogenous skin heating has been shown to attenuate
shivering heat production by an amount equivalent to the heat donated (34, 37, 64,
72). Passive warming was also effective in reducing cold discomfort. However, only
2/3 of the patients assigned to passive warming, whereas all patients assigned to
additional active warming presented a decrease in cold discomfort during transport.
This beneficial effect on thermal comfort by application of a chemical heat pad to
the upper torso is probably explained by a combination of reduction of shivering
thermogenesis and increased skin temperature.
32

Contrary to this study, two other randomized clinical trials found a decrease in
body core temperature with passive warming only, whereas with additional active
warming using either resistive heating blankets (38) or multiple chemical heat pads
(39), body core temperature was increased during transport. Because effective
passive warming requires adequate insulation materials in relation to ambient
conditions and intact shivering thermogenesis, differences regarding these factors
might explain differences between studies.
During field treatment additional active external warming rendered improved
thermal comfort, whereas passive warming alone did not (study III).
This is a difference compared to study II, where passive warming alone was
enough to reduce cold discomfort, even though additional active warming was even
more efficient. In both studies, the number of blankets used for passive warming
was about the same, but in study II, the evaluation was conducted during treatment
and transport in a heated ambulance or helicopter, whereas in study III evaluation
was conducted during treatment and transport in the outdoors. This probably
resulted in a greater cold stress during the evaluation period for patients in study III
than in study II, and thereby, probably also in increased shivering thermogenesis and
lower skin temperature. Although, an equal amount of insulation was present in both
studies, the realative cold stress seemed to be greater in study III. If cold stress
increases, demands on shivering thermogenesis during passive warming also
increases. In a scenario with trauma patients injured on the ski slopes, where
extensive shivering might be harmful, additional active warming seemed to be
beneficial.
Increased thermal comfort of patients in both study II and study III indicates a
beneficial effect on thermoregulation of additional active warming compared to
passive warming alone. This is most probably due to an increase of skin temperature
and to a reduction in demands on shivering thermogenesis and also consistent with
previous studies demonstrating that exogenous skin heating attenuates shivering by
an amount equivalent to the heat donated (34, 37, 64, 72).
Clinical randomized trials presented in this thesis indicate beneficial
thermophysiologic effects from active warming intervention. However, since body
core temperature remains stable if shivering and adequate passive warming are
intact, these results were based on subjective judgements of included patients.
Clinical studies, that, besides body core temperature, also investigate other objective
parameters and early predictors of cold induced stress, such as oxygen consumption,
are desirable.
Although probably due to different degrees of severity of injuries of included
patients as well as different heat sources and different amounts of passive warming,
33

esults from these and the few prior studies on active warming intervention in a
prehospital clinical setting (38, 39) are diverging. Therefore, and also because all
studies are relatively small, more and larger studies are desirable.
Future prehospital studies should also address more severely injured patients
suffering from moderate or severe hypothermia to evaluate effects of active
warming intervention regarding requirements of hospital treatment, morbidity and
mortality.
To have accurate measures to evaluate the thermal state of patients in the prehospital
setting is vitally important. In the field, especially in harsh ambient conditions, this
is often hard to achieve (14, 41). Thus, alternative measures to body core
temperature, skin temperature, and oxygen consumption, such as subjective
judgement scales for assessment of the patient’s thermal state, might be of
considerable importance in such scenarios, both for an initial assessment of the
patient and for evaluation of the treatment provided. Such assessment of the thermal
state of the patient might also be an early predictor of cold stress, and therefore may
be used to evaluate the risk of developing hypothermia.
It is important not to underestimate evaluation of the patient’s subjective
experience of medical care. Improved thermal comfort might have the potential of
relieving psychological stress such as the experience of pain and anxiety (9), which
might comprise a considerable physiological stress to the patient by increasing
respiratory and cardiovascular workload.
In a laboratory setting the test-retest stability of median CDS ratings showed
moderate to very good agreement. CDS ratings were generally somewhat higher
during test compared to retest, this difference might be a result of a decreased
sensitivity to the cold exposure from previous experience, and therefore being less
anxious, about exposure to the cold the second time, compared to the first time. This
tendency to habituation from repeated exposure might be a weakness of reliability of
the CDS. However, there was only one week between test and retest and if a longer
period would have passed between test and retest, reliability might have been even
better.
Criterion validity, when defined as the ability to detect changes in cold
discomfort due to increased cumulative cold stress from 30 minutes in - 20°C wind
still conditions, was good. CDS ratings proved to be statistical significantly
increased for every 30 minutes of cold exposure. However, during the last 20
34

minutes it seemed that CDS ratings did not increase as much as during the first 40
minutes. This tendency to habituation during exposure might be an indication of a
limitation to detect differences in cumulative cold stress when cold exposure is
protracted.
Bedford (91) and also Rholes (92) evaluated subjective judgement scales for
assessment of the thermal environment regarding construct validity, trying to
correlate subjective judgements of thermal comfort to objective measurements of the
thermal environment. Different from that, the CDS is evaluated for criterion validity,
defined as the ability to detect changes in cold discomfort due to changes in
cumulative cold stress, and therefore should be used to monitor changes in cold
discomfort over time. The CDS can not be used to measure and rank the level of
cold stress the patients are exposed to and that is the reason, why it is not possible to
make any comparisons between CDS ratings at different occasions.
This study is the first to evaluate reliability and criterion validity of a subjective
judgement scale for assessment of the thermal state of patients in an extreme cold
environment. Further studies including larger study populations, to confirm these
results would be desirable. Evaluating the CDS in different ambient conditions and
when using warming intervention would also be desirable.
35

Active external warming
Active external warming is recommended for protection from further cooling and
treatment of hypothermia in prehospital trauma care.
In non-shivering hypothermic subjects, heat sources applied on the chest and
upper back were effective to attenuate afterdrop, when providing high heat content
over a large surface area and effective to continue increasing body core temperature
when providing sustained high heat content.
In cold stressed, shivering trauma patients, adequate passive warming was
sufficient treatment to prevent afterdrop and slowly increase body core temperature.
Adequate passive warming also seemed sufficient to reduce cold discomfort, even
though additional active warming was even more efficient. When passive warming
was inadequate, additional active warming was required to reduce cold discomfort.
Prehospital monitoring
In a prehospital rescue scenario subjective judgement scales might be a valuable
measure for assessment of the thermal state of conscious patients.
The Cold Discomfort Scale, a subjective judgement scale for assessment of the
thermal state of patients in a cold environment seemed to be reliable, regarding testretest
stability, and valid, regarding ability to detect change in cumulative cold
stress.
36

During these years of research I have met and collaborated with so many great
people. All of you have, in one way or another, contributed to this thesis and I will
always be grateful for all your help. I will now, in Swedish, mention some of them,
who have been especially important along the way.
Jag vill börja med att rikta mitt största tack till Otto Henriksson, min gode vän
och kollega som jag jobbat tillsammans med under hela denna tid. Vi har upplevt
mycket, alltifrån fjällräddningsövningar, forskningsvistelser i Winnipeg och Lund,
otaliga resor till ambulansstationer och skidanläggningar samt vinterövningar i
Arvidsjaur och det har varit ett sant och stort nöje hela tiden.
Tack Otto!
Jag fortsätter där det hela började, nämligen i Linköping, där jag och Otto
tillbringade en stor del av vår studietid på Katastrofmedicinskt centrum (KMC) och
där vi lärde känna många trevliga och duktiga personer. Jag särskilt vill nämna några
som har blivit både mentorer och goda vänner.
Thomas Axelsson, Anders Sillén och Fawzi al-Ayoubi, äldrekursare, som
introducerade mig och Otto i katastrofmedicinen och forskningens värld. Ingrid
Björklund, lärare i katastrofmedicin, numera pensionär, som tog hand om både mig
och Otto som nykomlingar på KMC och som har gett oss många goda råd på vägen.
Nina Widfeldt, första handledaren, som på ett entusiastiskt och okomplicerat sätt
visade på att möjligheterna alltid är större än svårigheterna. Till sist men definitivt
inte minst i raden av betydelsefulla personer från Linköping; ToreVikström,
huvudhandledare under denna tid, chef på KMC, vars engagemang och stöd var den
avgörande anledningen till att jag och Otto påbörjade vår forskning.
Om man sedan fortsätter ut i den prehospitala verkligheten så skulle jag vilja
börja med att nämna Johnnie Bengtsson, som öppnade dörrarna till en organisation,
Fjällräddningen, som arbetar med sjukvård i dess allra mest prehospitala form, och
som har blivit en god vän under dessa år. Inom Fjällräddningen vill jag också tacka
sjukårdsinstruktörerna Staffan Isberg och Callis Blom, snö- och lavininstruktörerna
Kurt Övre och Rickard Svedjesten samt polischeferna Bengt-Göran Wiik och
Bengt Engwall för gott samarbete.
37

I en prehospital verklighet arbetar även i allra högsta grad skidpatrullerna på
Hamrafjället, i Hemavan och på Idre Fjäll samt ambulansbesättningarna i
Funäsdalen, Hede, Vemdalen, Svenstavik, Åre, Järpen, Krokom, Strömsund,
Backe, Gäddede, Skellefteå samt på ambulanshelikoptern i Östersund som alla
har gjort ett ovärderligt arbete i våra kliniska studier. Jag vill här framförallt tacka
Marie Nordgren, ansvarig för sjukvårdssutbildning inom SLAO (Svenska
Liftanläggningars Organisation), Jocke Nyberg, sjukvårdsansvarig i skidpatrullen
på Hamrafjället som tragiskt omkom i en lavinolycka denna vinter, Johan af
Ekenstam, sjukvårdsansvarig i skidpatrullen i Hemavan, Jonny Persson,
sjukvårdsansvarig i skidpatrullen på Idre Fjäll samt Peter Mattsson, Britt-Marie
Stolth, Anton Dahlmark, Christer Eriksson, Roland Olsson och Erik
Sandström, samtliga vid ambulanssjukvården i Jämtland och Härjedalen och
Magnus Strömqvist vid ambulanssjukvården i Västerbotten.
Praktiska aspekter på skydd mot kyla har jag haft det stora nöjet att få lära mig
mer om av min vän och kollega vid Försvarsmaktens Vinterenhet i Arvidsjaur, Tony
Gustafsson.
In want to thank Gordon Giesbrecht, for the great collaboration over these
years, especially for the two month stay in Winnipeg. It was a pleasure working in
the Cold Lab and I learned a lot about research methodology.
I Norge har Gunnar Vangberg och Haakon Nordseth varit viktiga
samarbetspartners.
Jag vill tacka Ingvar Holmér, Kalev Kuklane, Chuansi Gao och Faming
Wang vid Laboratoriet för termisk miljö i Lund för trevligt och mycket lärorikt
samarbete. Jag vill också rikta ett speciellt tack till Kalev som har lagt mängder med
tid, energi och tålamod på att ordna bästa tänkbara förutsättningar i laboratoriet.
Jag har under hela denna tid kännt ett stort stöd från min ordinarie arbetsplats,
Hallands sjukhus, Halmstad, där mina kollegor med intresse och engagemang
uppmuntrat mig till forskning och jag vill i detta sammanhang framför allt nämna
min chef på Medicinkliniken Berne Eriksson, schemaläggare Anders Bengtsson
och min kliniske handledare David Löfgren.
Jag har fått ovärderlig hjälp av Ulf Strömberg, Anders Holmén och Amir Baigi
FoUU-enheten, Hallands sjukhus med det komplexa och intressanta ämnet statistik.
38

Umeå universitet har varit basen för min forskning under doktorandstudierna
även om jag inte fysiskt befunnit mig där så ofta. Jag vill utöver forskargruppen på
Kunskapscentrum för katastrofmedicin speciellt tacka ett antal personer som jag inte
hade klarat mig utan under denna tid.
Anna Lundgren, som alltid på ett både engagerat och vänligt sätt har hjälpt till att
lösa alla tänkbara problem av administrativ karaktär.
Peter Naredi, handledare under de senaste två åren, som med stort engagemang har
tagit sig an den uppgiften och som med sin stora kunskap och erfarenhet har lämnat
ett mycket värdefullt bidrag till arbetet med såväl artiklar som denna avhandling.
Ulf Björnstig, huvudhandledare, som med sin stora kunskap och erfarenhet,
kombinerat med ett vänligt lugn och en aldrig sinande entusiasm har varit en sann
inspiratör under hela denna tid. Om Tore var en avgörande anledning till att jag och
Otto började med vår forskning så har Ulf varit en avgörande anledning till att vi
har fortsatt och kommer att fortsätta med forskning.
Jag hade tänkt att jag i detta avsnitt skulle hålla mig inom professionen, men det
går inte.
Jag vill avsluta med dem som står mig allra närmast, de som betyder allra mest,
de som utgör den allra största källan till inspiration, de som funnits med såväl innan,
som under tiden och framförallt på slutet av detta arbete, mina vänner och allra
viktigast, min familj.
39

This work was supported by the National Board of Health and Welfare, Emergency
Preparedness Unit.
40

1: Volunteer subject wearing light clothing in - 20°C (Study IV)
41

42
3: Chemical heat pad (study I)
4: Hot-water bag (study I)
5: Charcoal heater (study I)

6: The author in a prehospital setting
43

1. Arthurs Z, Cuadrado D, Beekley A, Grathwohl K, Perkins J, Rush R, et al. The
impact of hypothermia on trauma care at the 31st combat support hospital. American
journal of surgery. 2006;191(5):610-4.
2. Ireland S, Endacott R, Cameron P, Fitzgerald M, Paul E. The incidence and
significance of accidental hypothermia in major trauma--a prospective observational
study. Resuscitation. 2011;82(3):300-6.
3. Jurkovich GJ, Greiser WB, Luterman A, Curreri PW. Hypothermia in trauma
victims: an ominous predictor of survival. The Journal of trauma. 1987;27(9):1019-
24.
4. Martin RS, Kilgo PD, Miller PR, Hoth JJ, Meredith JW, Chang MC. Injuryassociated
hypothermia: an analysis of the 2004 National Trauma Data Bank. Shock.
2005;24(2):114-8.
5. Shafi S, Elliott AC, Gentilello L. Is hypothermia simply a marker of shock and
injury severity or an independent risk factor for mortality in trauma patients?
Analysis of a large national trauma registry. The Journal of trauma.
2005;59(5):1081-5.
6. Waibel BH, Schlitzkus LL, Newell MA, Durham CA, Sagraves SG, Rotondo MF.
Impact of hypothermia (below 36 degrees C) in the rural trauma patient. Journal of
the American College of Surgeons. 2009;209(5):580-8.
7. Wang HE, Callaway CW, Peitzman AB, Tisherman SA. Admission hypothermia
and outcome after major trauma. Critical care medicine. 2005;33(6):1296-301.
8. Aitken LM, Hendrikz JK, Dulhunty JM, Rudd MJ. Hypothermia and associated
outcomes in seriously injured trauma patients in a predominantly sub-tropical
climate. Resuscitation. 2009;80(2):217-23.
9. Robinson S, Benton G. Warmed blankets: an intervention to promote comfort for
elderly hospitalized patients. Geriatric nursing. 2002;23(6):320-3.
10. Swedish Meteorological and Hydrological Institute. Klimatdata.
http://www.smhi.se/klimatdata/meteorologi/temperatur; 2012 [2012-04-01].
11. Socialstyrelsen. Register of causes of death.
http://www.socialstyrelsen.se/register/dodsorsaksregistret; 2011 [2012-04-01].
44

12. Albiin N, Eriksson A. Fatal accidental hypothermia and alcohol. Alcohol and
alcoholism. 1984;19(1):13-22.
13. Giesbrecht GG. Hypothermia, frostbite, and other cold injuries : prevention,
survival, rescue and treatment. Seattle, WA: Mountaineers Books; 2006.
14. Socialstyrelsen. Hypothermia, cold injuries and cold water near drowning.
Stockholm: The National Board of Health and Welfare (Socialstyrelsen); 2002.
15. Danzl DF. Accidental hypothermia. In: Auerbach PS, editor. Wilderness medicine.
Philadelphia: Mosby Elsevier; 2007. p. 125-59.
16. Mallet ML. Pathophysiology of accidental hypothermia. QJM : monthly journal of
the Association of Physicians. 2002;95(12):775-85.
17. Beilman GJ, Blondet JJ, Nelson TR, Nathens AB, Moore FA, Rhee P, et al. Early
hypothermia in severely injured trauma patients is a significant risk factor for
multiple organ dysfunction syndrome but not mortality. Annals of surgery.
2009;249(5):845-50.
18. Helm M, Lampl L, Hauke J, Bock KH. [Accidental hypothermia in trauma patients.
Is it relevant to preclinical emergency treatment?]. Der Anaesthesist.
1995;44(2):101-7. Akzidentelle Hypothermie bei Traumapatienten. Von Relevanz
bei der praklinischen Notfallversorgung?
19. Steinemann S, Shackford SR, Davis JW. Implications of admission hypothermia in
trauma patients. The Journal of trauma. 1990;30(2):200-2.
20. Husum H, Olsen T, Murad M, Heng YV, Wisborg T, Gilbert M. Preventing postinjury
hypothermia during prolonged prehospital evacuation. Prehospital and
disaster medicine. 2002;17(1):23-6.
21. Langhelle A, Lockey D, Harris T, Davies G. Body temperature of trauma patients on
admission to hospital: a comparison of anaesthetised and non-anaesthetised patients.
Emergency medicine journal : EMJ. 2012;29(3):239-42.
22. Seekamp A, Ziegler M, Van Griensven M, Grotz M, Regel G. The role of
hypothermia in trauma patients. European journal of emergency medicine : official
journal of the European Society for Emergency Medicine. 1995;2(1):28-32.
23. Hildebrand F, Giannoudis PV, van Griensven M, Chawda M, Pape HC.
Pathophysiologic changes and effects of hypothermia on outcome in elective surgery
and trauma patients. American journal of surgery. 2004;187(3):363-71.
45

24. Tisherman SA. Hypothermia and injury. Current opinion in critical care.
2004;10(6):512-9.
25. Tsuei BJ, Kearney PA. Hypothermia in the trauma patient. Injury. 2004;35(1):7-15.
26. Johnson CF. Oxford handbook of expedition and wilderness medicine. Oxford:
Oxford University Press; 2008.
27. National Association of Emergency Medical Technicians (U.S.). Pre-Hospital
Trauma Life Support Committee., American College of Surgeons. Committee on
Trauma. PHTLS : Prehospital Trauma Life Support. 7th ed. St. Louis, Mo.: Mosby
Jems/Elsevier; 2011. 618 p. p.
28. Durrer B, Brugger H, Syme D, International Commission for Mountain Emergency
M. The medical on-site treatment of hypothermia: ICAR-MEDCOM
recommendation. High altitude medicine & biology. 2003;4(1):99-103.
29. Giesbrecht GG. Prehospital treatment of hypothermia. Wilderness & environmental
medicine. 2001;12(1):24-31.
30. Mills WJ. Field care of the hypothermic patient. International journal of sports
medicine. 1992;13 Suppl 1:S199-202.
31. Peng RY, Bongard FS. Hypothermia in trauma patients. Journal of the American
College of Surgeons. 1999;188(6):685-96.
32. Department of Health and Social Services;Alaska So. State of Alaska, Cold injuries
gudielines.
http://www.hss.state.ak.us/dph/emergency/ems/assets/Downloads/AKColdInj2005.p
df; 2005 [2012-01-04].
33. Allen PB, Salyer SW, Dubick MA, Holcomb JB, Blackbourne LH. Preventing
hypothermia: comparison of current devices used by the US Army in an in vitro
warmed fluid model. The Journal of trauma. 2010;69 Suppl 1:S154-61.
34. Giesbrecht GG, Bristow GK, Uin A, Ready AE, Jones RA. Effectiveness of three
field treatments for induced mild (33.0 degrees C) hypothermia. Journal of applied
physiology (Bethesda, Md : 1985). 1987;63(6):2375-9.
35. Greif R, Rajek A, Laciny S, Bastanmehr H, Sessler DI. Resistive heating is more
effective than metallic-foil insulation in an experimental model of accidental
hypothermia: A randomized controlled trial. Annals of emergency medicine.
2000;35(4):337-45.
46

36. Hultzer MV, Xu X, Marrao C, Bristow G, Chochinov A, Giesbrecht GG. Prehospital
torso-warming modalities for severe hypothermia: a comparative study
using a human model. Cjem. 2005;7(6):378-86.
37. Sterba JA. Efficacy and safety of prehospital rewarming techniques to treat
accidental hypothermia. Annals of emergency medicine. 1991;20(8):896-901.
38. Kober A, Scheck T, Fulesdi B, Lieba F, Vlach W, Friedman A, et al. Effectiveness
of resistive heating compared with passive warming in treating hypothermia
associated with minor trauma: a randomized trial. Mayo Clinic proceedings Mayo
Clinic. 2001;76(4):369-75.
39. Watts DD, Roche M, Tricarico R, Poole F, Brown JJ, Jr., Colson GB, et al. The
utility of traditional prehospital interventions in maintaining thermostasis.
Prehospital emergency care : official journal of the National Association of EMS
Physicians and the National Association of State EMS Directors. 1999;3(2):115-22.
40. Berger RL. Nazi science--the Dachau hypothermia experiments. The New England
journal of medicine. 1990;322(20):1435-40.
41. Crawshaw LI. Thermoregulation. In: Auerbach PS, editor. Wilderness medicine.
Philadelphia: Mosby Elsevier; 2007. p. 110-24.
42. Parsons KC. Human thermal physiology and thermoregulation. In: Parsons KC,
editor. Human thermal environments : the effects of hot, moderate and cold
environments on human health, comfort and performance. 2nd ed. ed. London:
Taylor & Francis; 2003. p. 31-48.
43. Webb P. The physiology of heat regulation. The American journal of physiology.
1995;268(4 Pt 2):R838-50.
44. Holmer I. Protection against cold. In: Shishoo R, editor. Textiles in sport.
Cambridge, England: Woodhead Pub; 2005. p. 262-86.
45. Parsons KC. Human thermal environments. In: Parsons KC, editor. Human thermal
environments : the effects of hot, moderate and cold environments on human health,
comfort and performance. 2nd ed. ed. London: Taylor & Francis; 2003. p. 1-30.
46. Eyolfson DA, Tikuisis P, Xu X, Weseen G, Giesbrecht GG. Measurement and
prediction of peak shivering intensity in humans. European journal of applied
physiology. 2001;84(1-2):100-6.
47

47. Flouris AD. Functional architecture of behavioural thermoregulation. European
journal of applied physiology. 2011;111(1):1-8.
48. Kuno S OH, Nakahara N. A two-dimensional model expressing thermal sensation in
transitional conditions. ASHRAE Transactions. 1987(93):396-406.
49. Flouris AD CS. Human conscious response to thermal input is adjusted to changes
in mean body temperature. British journal of sports medicine. 2008(43):199-203.
50. Attia M EP. Thermal pleasantness sensation: an indicator of thermal stress.
European journal of applied physiology. 1982(50):55-70.
51. Attia M EP, Hildebrandt G. Quantification of thermal comfort parameters using a
behavioural indicator. Physiol Behav. 1980(24):901-9.
52. H H. Thermoreception and Temperature Regulation. London, UK: Academic Press;
1981.
53. Parsons KC. Psychological responses. In: Parsons KC, editor. Human thermal
environments : the effects of hot, moderate and cold environments on human health,
comfort and performance. 2nd ed. ed. London: Taylor & Francis; 2003. p. 49-70.
54. Aslam AF, Aslam AK, Vasavada BC, Khan IA. Hypothermia: evaluation,
electrocardiographic manifestations, and management. The American journal of
medicine. 2006;119(4):297-301.
55. Vassallo SU DK, Hoffman RS, Slater W, Goldfrank LR. A prospective evaluation
of the electrographic manifestations of hypothermia. Academic emergency
medicine: official journal of the Society for Academic Emergency Medicine.
1999(6):1121-6.
56. Shaikh N MS, Gowda RM, Kahn IA. Electrographic features of hypothermia
hypothermia. Cardiology. 2005(103):118-9.
57. Oksa J. Neuromuscular performance limitations in cold. International journal of
circumpolar health. 2002;61(2):154-62.
58. Rintamaki H. Performance and energy expenditure in cold environments. Alaska
medicine. 2007;49(2 Suppl):245-6.
59. Johnston TD, Chen Y, Reed RL, 2nd. Functional equivalence of hypothermia to
specific clotting factor deficiencies. The Journal of trauma. 1994;37(3):413-7.
48

60. Wolberg AS, Meng ZH, Monroe DM, 3rd, Hoffman M. A systematic evaluation of
the effect of temperature on coagulation enzyme activity and platelet function. The
Journal of trauma. 2004;56(6):1221-8.
61. Rajagopalan S, Mascha E, Na J, Sessler DI. The effects of mild perioperative
hypothermia on blood loss and transfusion requirement. Anesthesiology.
2008;108(1):71-7.
62. Schmied H, Kurz A, Sessler DI, Kozek S, Reiter A. Mild hypothermia increases
blood loss and transfusion requirements during total hip arthroplasty. Lancet.
1996;347(8997):289-92.
63. Winkler M, Akca O, Birkenberg B, Hetz H, Scheck T, Arkilic CF, et al. Aggressive
warming reduces blood loss during hip arthroplasty. Anesthesia and analgesia.
2000;91(4):978-84.
64. Giesbrecht GG, Sessler DI, Mekjavic IB, Schroeder M, Bristow GK. Treatment of
mild immersion hypothermia by direct body-to-body contact. Journal of applied
physiology (Bethesda, Md : 1985). 1994;76(6):2373-9.
65. Giesbrecht GG, Goheen MS, Johnston CE, Kenny GP, Bristow GK, Hayward JS.
Inhibition of shivering increases core temperature afterdrop and attenuates
rewarming in hypothermic humans. Journal of applied physiology (Bethesda, Md :
1985). 1997;83(5):1630-4.
66. Goheen MS, Ducharme MB, Kenny GP, Johnston CE, Frim J, Bristow GK, et al.
Efficacy of forced-air and inhalation rewarming by using a human model for severe
hypothermia. Journal of applied physiology (Bethesda, Md : 1985).
1997;83(5):1635-40.
67. Hamilton RS PB. The diagnosis and treatment of hypothermia by mountain rescue
teams: a survey. Wilderness Environ Med. 1996(7):28-37.
68. Giesbrecht GG, Bristow GK. The convective afterdrop component during
hypothermic exercise decreases with delayed exercise onset. Aviation, space, and
environmental medicine. 1998;69(1):17-22.
69. TG L. Hypothermia: Killer of the Unprepared. Portland: Mazamas; 1975.
70. Mills W J HP, Schoene RB, Roach R, Mills W I. Treatment of Hypothermia in the
Field. In: Sutton JR HC, Coates G, editor. Hypoxia and Cold. New York: Praeger;
1987. p. 271-85.
49

71. Collis ML. Treatment of the Hypothermic Subject. In: Greene B, editor. Cold Water
Symposium; Toronto: Life Saving Society Canada; 1976. p. 31-2.
72. Harnett RM, O'Brien EM, Sias FR, Pruitt JR. Initial treatment of profound
accidental hypothermia. Aviation, space, and environmental medicine.
1980;51(7):680-7.
73. Collis ML, Steinman AM, Chaney RD. Accidental hypothermia: an experimental
study of practical rewarming methods. Aviation, space, and environmental
medicine. 1977;48(7):625-32.
74. Danzl DF, Pozos RS, Auerbach PS, Glazer S, Goetz W, Johnson E, et al.
Multicenter hypothermia survey. Annals of emergency medicine. 1987;16(9):1042-
55.
75. Hayward JS, Steinman AM. Accidental hypothermia: an experimental study of
inhalation rewarming. Aviation, space, and environmental medicine.
1975;46(10):1236-40.
76. Miller JW, Danzl DF, Thomas DM. Urban accidental hypothermia: 135 cases.
Annals of emergency medicine. 1980;9(9):456-61.
77. Mekjavic IB, Eiken O. Inhalation rewarming from hypothermia: an evaluation in -20
degrees C simulated field conditions. Aviation, space, and environmental medicine.
1995;66(5):424-9.
78. Romet TT, Hoskin RW. Temperature and metabolic responses to inhalation and bath
rewarming protocols. Aviation, space, and environmental medicine. 1988;59(7):630-
4.
79. Brandon Bravo LJ. Thermodynamic modelling of hypothermia. European journal of
emergency medicine : official journal of the European Society for Emergency
Medicine. 1999;6(2):123-7.
80. Silbergleit R, Satz W, Lee DC, McNamara RM. Hypothermia from realistic fluid
resuscitation in a model of hemorrhagic shock. Annals of emergency medicine.
1998;31(3):339-43.
81. Mekjavic IB, Rempel ME. Determination of esophageal probe insertion length
based on standing and sitting height. Journal of applied physiology (Bethesda, Md :
1985). 1990;69(1):376-9.
50

82. Cooper KE, Kenyon JR. A comparison of temperatures measured in the rectum,
oesophagus, and on the surface of the aorta during hypothermia in man. The British
journal of surgery. 1957;44(188):616-9.
83. Hayward JS, Eckerson JD, Kemna D. Thermal and cardiovascular changes during
three methods of resuscitation from mild hypothermia. Resuscitation. 1984;11(1-
2):21-33.
84. Walpoth BH, Galdikas J, Leupi F, Muehlemann W, Schlaepfer P, Althaus U.
Assessment of hypothermia with a new "tympanic" thermometer. Journal of clinical
monitoring. 1994;10(2):91-6.
85. Webb P. Heat storage and body temperature during cooling and rewarming.
European journal of applied physiology and occupational physiology.
1993;66(1):18-24.
86. Duberg T LC, Holmberg H. Ear thermometer not an adequate alternative to rectal
thermometer. A comparativestudy shows big temperature discrepancies of ear
temperature measurements. Lakartidningen. 2007;May 9-15(104(19)):1479-82.
87. Ducharme MB, Frim J, Bourdon L, Giesbrecht GG. Evaluation of infrared tympanic
thermometers during normothermia and hypothermia in humans. Annals of the New
York Academy of Sciences. 1997;813:225-9.
88. Edling L CR, Magnuson A, Holmberg H. . Läkartidningen. 2009 ;:. Rectal
thermometers still best for body temperature measurement. Comparative study of
tympanic and oral thermometers. Lakartidningen. 2009;Oct 14-20(106(42)):2680-3.
89. Lilly JK, Boland JP, Zekan S. Urinary bladder temperature monitoring: a new index
of body core temperature. Critical care medicine. 1980;8(12):742-4.
90. ISO-10551. Ergonomics of the thermal environment - Assessment of the influence
of the thermal environment using subjective judgement scales. Geneva: International
Standards Organisation; 2001.
91. Bedford. The warmth factor in comfort at work. MRC Industrial Health Board
Report HMSO. London, UK: 1936.
92. Rohles. The nature of thermal comfort for sedentary man. ASHRAE Transactions,
ashrae org. 1971;77 (1):239-46.
51

PRELIMINARY REPORT
FIELD TORSO-WARMING MODALITIES
ACOMPARATIVE STUDY USING A HUMAN MODEL
J. Peter Lundgren, MD, Otto Henriksson, MD, Thea Pretorius, MSc, Farrell Cahill, BKin,
Gerald Bristow, MD, Alecs Chochinov, MD, Alexander Pretorius, MD, Ulf Bjornstig, MD, PhD,
Gordon G. Giesbrecht, PhD
ABSTRACT
Objective. To compare four field-appropriate torso-warming
modalities that do not require alternating-current (AC)
electrical power, using a human model of nonshivering hypothermia.
Methods. Five subjects, serving as their own controls,
were cooled four times in 8 ◦ C water for 10–30 minutes.
Shivering was inhibited by buspirone (30 mg) taken
orally prior to cooling and intravenous (IV) meperidine
(1.25 mg/kg) at the end of immersion. Subjects were hoisted
out of the water, dried, and insulated and then underwent
120 minutes of one of the following: spontaneous warming
only; a charcoal heater on the chest; two flexible hotwater
bags (total 4 liters of water at 55 ◦ C, replenished every
20 minutes) applied to the chest and upper back; or
two chemical heating pads applied to the chest and upper
back. Supplemental meperidine (maximum cumulative dose
of 3.5 mg/kg) was administered as required to inhibit shivering.
Results. The postcooling afterdrop (i.e., the continued
decrease in body core temperature during the early period
of warming), compared with spontaneous warming (2.2 ◦ C),
Received December 3, 2008, from the Division of Surgery, Department
of Surgery and Perioperative Sciences (JPL, OH, UB), Ume˚a
University, Ume˚a, Sweden; the Laboratory for Exercise and Environmental
Medicine (TP, FCBK, GB, GGG), Department of Anesthesia,
Faculty of Medicine (GB, GGG), and Department of Emergency
Medicine, Faculty of Medicine (AC), University of Manitoba, Winnipeg,
Manitoba, Canada; and the Department of Anesthesia, Grace
Hospital (AP), Winnipeg, Manitoba, Canada. Revision received
January 14, 2009; accepted for publication January 30, 2009.
Supported by the Natural Sciences and Engineering Research Council
of Canada and the National Board of Health and Welfare and
Jamtland County Council, Sweden.
The authors report no conflicts of interest. The authors alone are responsible
for the content and writing of the paper.
Address correspondence and reprint requests to: Dr. J. Peter Lundgren,
Division of Surgery, Department of Surgery and Perioperative
Sciences, Ume˚a University, SE-90185 Ume˚a, Sweden. e-mail: lundgren.jp@hotmail.com
doi: 10.1080/10903120902935348
371
was less for the chemical heating pads (1.5◦C) and the hotwater
bags (1.6◦C, p < 0.05) and was 1.8◦C for the charcoal
heater. Subsequent core rewarming rates for the hot-water
bags (0.7◦C/h) and the charcoal heater (0.6◦C/h) tended to be
higher than that for the chemical heating pads (0.2◦C/h) and
were significantly higher than that for spontaneous warming
rate (0.1◦C/h, p < 0.05). Conclusion. In subjects with
shivering suppressed, greater sources of external heat were
effective in attenuating core temperature afterdrop, whereas
sustained sources of external heat effectively established core
rewarming. Depending on the scenario and available resources,
we recommend the use of charcoal heaters, chemical
heating pads, or hot-water bags as effective means for treating
cold patients in the field or during transport to definitive
care. Key words: hypothermia; body temperature regulation;
rewarming; emergency medical services
PREHOSPITAL EMERGENCY CARE 2009;13:371–378
INTRODUCTION
A major concern in the prehospital care of patients exposed
to a cold environment, either through cold air
or cold water immersion, is to reduce cold stress and
avoid further heat loss, thereby diminishing the risk of
cold-induced cardiac or respiratory failure. Initial measures
should be taken to insulate the patient from the
ground, remove wet clothing if possible, and contain
endogenous heat production within a vapor barrier
and adequate wind- and waterproof insulation. Application
of some form of exogenous heat should then
be considered to reduce the depth and duration of the
core temperature (Tco) afterdrop (i.e., the continued decrease
in body temperature during the early period of
rewarming) and establish a steady moderate rate of Tco
rewarming. 1−7
For the mildly hypothermic victim (Tco = 35–32 ◦ C)
who is physiologically stable, spontaneous warming
due to shivering heat production provides reduction
of afterdrop and establishes a safe and efficient
rewarming rate. 2,3,8−13 Several studies on mildly

372 PREHOSPITAL EMERGENCY CARE JULY/SEPTEMBER 2009 VOLUME 13 / NUMBER 3
hypothermic shivering subjects have found that exogenous
skin heating attenuates shivering heat production
by an amount equivalent to the heat donated. 8−13
Accordingly, body-to-body warming, 9,11 forced-air
warming, 10 application of electrical and hot-waterperfused
heating pads, 11,12 and charcoal heaters 8,13
have produced reduction of afterdrop and established
rewarming rates similar to that of spontaneous shivering
alone. Thus, in a mildly hypothermic shivering
victim, external warming generally does not decrease
afterdrop or increase the rewarming rate; however, it
might provide other advantages, including increased
comfort, decreased cardiac work, and preserved substrate
availability.
When shivering is diminished or absent in moderate
(Tco = 32–28 ◦ C) to severe (Tco < 28 ◦ C) hypothermia or
otherwise impaired because of the overall medical condition
of the patient (i.e., old age, alcohol or drug ingestion,
head or spinal injury, severe trauma, or depleted
metabolic energy substrates), some form of exogenous
external or internal heat is required; otherwise, afterdrop
will continue and little or no rewarming will occur.
This was demonstrated using a human model of
nonshivering hypothermia, where meperidine was administered
to inhibit shivering in mildly hypothermic
subjects. 14−16 With metabolic and thermal responses
similar to those in actual severe hypothermic conditions,
subjects using spontaneous warming only experienced
an increased afterdrop, with rewarming either
attenuated or eliminated compared with that in subjects
receiving an exogenous heat supply.
In a summary of survey responses from 41 Mountain
Rescue Association teams, the most common protocols
for treatment of hypothermia were chemical heating
pads (46%), body-to-body warming (39%), and hotwater
bottles applied to the trunk (32%). 17 Although
chemical heating pads and hot-water bottles are commonly
used and recommended, scientific verification
of their effectiveness is minimal or nonexistent. In fact,
these measures are recommended in some prehospital
treatment guidelines 4,6 but discouraged in others. 7,18
Effective prehospital field warming is considered of
utmost importance to improve the medical condition
of severely hypothermic patients on admission to the
emergency department. 1−7 It is therefore important to
quantify the thermal effectiveness of those modalities
that could be used in the field by laypersons, search
and rescue (SAR) personnel, or the emergency medical
services (EMS) system.
We therefore decided to use a human model for nonshivering
hypothermia 14,15 to evaluate the thermal effectiveness
of chemical heating pads and hot-water
bottles. To increase the surface area in contact with the
skin, flexible nylon water bags were used instead of
rigid bottles. For comparative reasons, the previously
evaluated charcoal heater and spontaneous warming
were selected. These torso-warming modalities, being
Subject Gender
TABLE 1. Characteristics of the Subjects
Age,
years
Height,
cm
Weight,
kg
Body
fat ∗ ,%
BSA † ,
m 2
BMI ‡ ,
kg/m 2
1 Male 35 174 78 26 1.9 26
2 Male 27 173 91 29 2.1 31
3 Male 40 175 110 30 2.2 36
4 Male 29 180 73 21 1.9 22
5 Male 28 175 68 13 1.8 22
Mean 32 175 84 24 2.0 27
SD 6 3 17 7 0.2 6
∗ 19
Calculated according to Durnin and Womersley.
† 20
Calculated according to Dubois and Dubois.
‡ 21
Calculated according to McArdle et al.
BSA = body surface area; BMI = body mass index; SD = standard deviation.
portable and requiring no external electrical power, are
all suited for prehospital field care.
METHODS
Design, Setting, and Subjects
The study was approved by the Education/Nursing
Research Ethics Board of the University of Manitoba.
Five male subjects volunteered for participation (Table
1). They had no history of allergy to or current use of
narcotics. Written informed consent was obtained from
all patients. Studies were conducted in the Laboratory
for Exercise and Environmental Medicine at the University
of Manitoba in February and March 2006.
Monitoring
Esophageal temperature (Tes), 22,23 oxygen consumption
(Vo2), respiratory exchange ratio (RER), electrocardiography
(ECG), heart rate (HR), and arterial oxygen
saturation were continuously monitored and recorded
during the trials as described previously. 14,15 Endogenous
heat production (M) in watts (W) was calculated
from VO2 and RER according to the following equation:
M(W)= VO2 (L/ min)
× 69.7(4.686 + [(RER − 0.707) × 1.232])
where M represents heat production and W represents
watts.
Skin heat transfer (Qskin; W·m −2 ) and skin temperature
( ◦ C) were measured from 12 sites using thermal
flux transducers (Concept Engineering, Old Saybrook,
CT). Skin heat transfer for a specific body part
(Qbody part; W) was then calculated using heat flux values
for each transducer (W/m 2 ) according to the following
equation:
Qbody part(W) = transducer flux (W/m 2 ) × BSA(m 2 )
× body part percentage

Lundgren et al. FIELD TORSO-WARMING MODALITIES 373
where BSA represents body surface area; the body part
percentage was estimated according to Layton et al. 24
Intravenous (IV) access was obtained in the right
forearm or hand for the purpose of drug and/or saline
administration.
Protocol
Each subject served as his or her own control for comparative
evaluation of each of the warming modalities,
and was cooled at the same time of day on four
separate occasions. The order of the trials followed
a balanced design. Subjects dressed in a bathing suit
and sat quietly at an ambient temperature of approximately
22 ◦ C for 10 minutes of baseline data collection
after monitors were applied. To enhance the effect of
meperidine, the subjects took buspirone (30 mg orally)
during the instrumentation period. They were then immersed
to the level of the sternal notch in a stirred water
bath. The temperature of the water was lowered,
by rapid inflow of 2 ◦ C water from a large reservoir,
from 21 ◦ Cto8 ◦ C over a period of 5 minutes. Subjects
were immersed for 10 to 30 minutes depending
on their body mass, with their immersion time being
based on the results of prior pilot studies. Immersion
time was the same for all conditions for each subject
and was limited by the highest amount of body cooling
that could occur for which the subject’s shivering
could be successfully inhibited by the prescribed maximal
dose of meperidine.
During the last 10 minutes of immersion, subjects
were given 1.25 mg/kg of IV meperidine (diluted
in five 2-mL aliquots and injected over successive 2minute
intervals). Subjects were then hoisted out of the
water, towel dried, and placed in a sleeping bag, with
the head covered, for 120 minutes of warming. Postimmersion
supplemental injections of meperidine to
a maximum cumulative dose of 3.5 mg/kg were administered
based on Vo2 and the subject’s sensation of
shivering in order to maintain shivering suppression.
Each trial was terminated after 120 minutes of warming,
a duration sufficient to establish a steady rate of
core temperature change. Subjects were then immersed
in 42 ◦ C water until their Tes rose to a normothermic
level.
Warming Modalities
No exogenous heat source was used in the spontaneous
warming trials. The materials and protocols for
the warming modalities are as described in the sections
that follow.
Charcoal Heater
The heater consisted of a combustion chamber, charcoal
fuel, and a branched, reinforced, but flexible, heat-
FIGURE 1. Top:charcoalheaterinuse;middle: hot-water bags in use;
and bottom: chemical heating pads in use.
ing duct (Normeca AS, Oslo, Norway) and produced
250 W of heat (1,800 kJ over 120 minutes). The combustion
chamber was placed on the subject’s chest and
the heating ducts were applied dorsally over the shoulders,
and then anteriorly under the axillae to cross over
the lower chest (Fig. 1, top). The total skin contact surface
area of the chamber (23 × 12 × 6 cm, 1,100 g) and

374 PREHOSPITAL EMERGENCY CARE JULY/SEPTEMBER 2009 VOLUME 13 / NUMBER 3
ducts was about 1,500 cm 2 . The heater was ignited and
set to the “high” setting 15–30 minutes before being
applied to the subject. The heater could produce maximum
heat for approximately eight hours. Subjects laid
their hands on the heater during warming.
Hot-Water Bags
Two 6-liter flexible water bags (Mountain Safety Research,
Seattle, WA) were each filled with 2 liters of
55 ◦ C water. This volume of water was used because
a total of 4 liters was considered a realistic volume that
could be heated in the field, and, filled with this volume
of water, the water bag lay flat, allowing virtually
all of one side of it to contact the skin. Each bag (45 ×
25 cm, 120 g) had a skin-contact surface area of about
1,100 cm 2 . A towel was placed between each bag and
the skin to prevent burn injury. Subjects lay with one
of the bags placed under their upper back, while the
other bag was placed on the upper chest (Fig. 1, middle).
Every 20 minutes, both bags were refilled with
55 ◦ C water. Subjects laid their hands on the top of the
chest water bag as soon as it was comfortable.
Chemical Heating Pads
Two chemical heating pads (Dorcas AB, Skattkarr,
Sweden) were activated 2 minutes prior to use. Each
pad (42 × 25 × 2 cm, 1,400 g) had a skin contact surface
area of about 1,100 cm2 . Subjects lay with one
of the pads placed under their upper back, while the
other pad was placed on the upper chest (Fig. 1, bottom).
Surface temperature on the skin side of the pads
reached approximately 50◦C within 2 minutes after activation
and then gradually declined. Initially a towel
was placed between each pad and the skin to prevent
burn injury. The towel was removed after 30 minutes,
as pad surface temperature had decreased to a
level where skin temperature could remain below the
threshold (∼43◦C) for burn injury. 25 Subjects laid their
hands on the chest pad during warming.
Data Analysis
Data were compared using a repeated-measures analysis
of variance (ANOVA) with post hoc analysis with
Fisher’s protected least significant difference (PLSD)
test to identify significant differences. Results are reported
as means ± standard deviation (SD), and p <
0.05 was the threshold defined for statistically significant
differences.
RESULTS
Endogenous Heat Production
Metabolic heat production increased from 116 ± 17 W
during baseline to 195 ± 51 W during the last 10
minutes before meperidine injection. Meperidine suppressed
shivering, with heat production returning to
114 ± 21 W during the first 40 minutes after cooling
and then subsequently falling to 97 ± 17 W throughout
the remaining 80 minutes of warming. There were
no differences in heat production for the different conditions.
Heart Rate, Respiratory Rate, and Skin
Temperature
Heart rate and respiratory rate increased during cooling
from baseline values of 74 ± 13 beats/min and
19 ± 5 breaths/min to 87 ± 19 beats/min and 21
± 7 breaths/min, respectively, just before meperidine
administration. Postimmersion heart rate and respiratory
rate declined to 66 ± 13 beats/min and 17 ± 6
breaths/min, respectively. There were no significant
differences between the different conditions.
Skin temperature on the chest and upper back
reached maximum values of 41.6 ◦ C for the charcoal
heater, 42.3 ◦ C for the hot-water bags, and 42.8 ◦ Cfor
the chemical heating pads.
Body Core Temperature
There were no significant differences in initial cooling
rate (−10 to 0 min) for the different conditions
(Table 2 and Fig. 2). The postcooling afterdrop compared
with spontaneous warming (2.2◦C) was significantly
less for the chemical heating pads (1.5◦C) and
the hot-water bags (1.6◦C, p < 0.05), but not for the
charcoal heater (1.8◦C). The time to Tes nadir was significantly
less for all other modalities compared with
spontaneous warming (p < 0.05). Subsequent core rewarming
rates for the hot-water bags (0.7◦C/h) and the
charcoal heater (0.6◦C/h) tended to be higher than that
for the chemical heating pads (0.2◦C/h) and were significantly
greater than that for spontaneous warming
(0.1◦C/h, p < 0.05).
Exogenous Heat Delivery
The heat gain during active warming on the chest
and upper back (each being 9% of body surface area)
is shown in Figure 3. The charcoal heater provided
a steady heat gain primarily to the chest. The water
bottles donated heat to both the chest and upper
back, with heat transfer being slightly greater on the
back than the chest; the amount of heat transferred
transiently increased each time water was replaced.
The chemical heating pads also donated heat to both
the chest and upper back, with the amount of heat
transferred decreasing for the initial 30 minutes. Once
the towels were removed, heat transfer transiently increased
and then again decreased to minimal levels

Lundgren et al. FIELD TORSO-WARMING MODALITIES 375
Modality
Cooling Rate
(−10 to 0 min),
◦ C/h
TABLE 2. Core Temperature Responses ∗
Afterdrop
Amount,
◦ C
Time to
Tes Nadir,
min
Rewarming
Rate (60–120 min),
◦ C/h
Change in
Tes
(0–120 min), ◦C Charcoal heater −0.8 (1.4) −1.8 (0.4) 51 (30) † 0.6 (0.5) † −1.1 (0.6) †
Hot-water bags −0.8 (1.3) −1.6 (0.2) † 44 (23) † 0.7 (0.3) † −0.6 (0.6) †‡
Chemical heating pads −0.4 (1.6) −1.5 (0.4) † 46 (23) † 0.2 (0.3) −0.9 (0.7) †
Spontaneous warming −0.9 (1.7) −2.2 (0.3) 88 (38) 0.1 (0.8) −1.7 (0.8)
∗Values are mean ± standard deviation.
† Significantly different from spontaneous warming (p < 0.05).
‡ Significantly different from charcoal heater (p < 0.05).
Tes = esophageal temperature.
Change in core temperature (°C)
Change in core temperature (°C)
1,0
0,0
-1,0
-2,0
1,0
0,0
-1,0
-2,0
Start
cooling
-3,0
-40 -20 0 20 40
Time (min)
60 80 100 120
Start
cooling
Start active
warming
First dose of
meperidine
Start active
warming
First dose of
meperidine
Hot-Water Bags
Charcoal Heater
-3,0
-40 -20 0 20 40
Time (min)
60 80 100 120
b
c
Change in core temperature (°C)
Change in core temperature (°C)
1,0
0,0
-1,0
-2,0
-3,0
-40 -20 0 20 40
Time (min)
60 80 100 120
1,0
0,0
-1,0
-2,0
Start
cooling
Start active
warming
Start
cooling
First dose of
meperidine
Start active
warming
First dose of
meperidine
Chemical Heating
Pads
-3,0
-40 -20 0 20 40
Time (min)
60 80 100 120
a
b
Spontaneous
Warming
FIGURE 2. Change in esophageal temperature (Tes) during four warming protocols (mean, n = 5). a Afterdrop amount and b time to Tes nadir less
than spontaneous warming; c final Tes significantly greater than spontaneous warming; d final Tes significantly greater than charcoal heater (p <
0.05). CP = chemical heating pads; HP = charcoal heater; SP = spontaneous warming; WB = hot-water bags.
c

376 PREHOSPITAL EMERGENCY CARE JULY/SEPTEMBER 2009 VOLUME 13 / NUMBER 3
Chest Cutaneous Heat Gain (W)
75
50
25
0
SP CP WB HP
-25
0 20 40 60
Time (min)
80 100 120
Upper Back Cutaneous Heat Gain (W)
75
50
25
0
SP CP WB HP
-25
0 20 40 60
Time (min)
80 100 120
FIGURE 3. Cutaneous heat gain on the chest and upper back during four warming protocols (mean, n = 5). CP = chemical heating pads; HP =
charcoal heater; SP = spontaneous warming; WB = hot-water bags (note the sawtooth pattern due to replenishing the water).
over the next 90 minutes. During spontaneous warming
there was a small, continuous steady heat loss from
the upper torso. Total cumulative energy transfer to the
chest and upper back during 120 minutes of warming
was 536 ± 56 kJ for the hot-water bags, 246 ± 71 kJ for
the chemical heating pads, 230 ± 50 kJ for the charcoal
heater, and −50 ± 28 kJ for spontaneous warming, and
this was significantly different for each of the warming
modalities except chemical heating pads vs. charcoal
heater (p < 0.05).
DISCUSSION
This study was unique in that it used a human model
for nonshivering hypothermia to evaluate relative efficacy
of torso-warming procedures that could be used
in the field and during transport to the hospital. Hotwater
bags and chemical heating pads, which to our
knowledge have not been quantified before, reduced
both the amount and the duration of the subsequent
afterdrop following removal from cold stress. The
charcoal heater had little effect on afterdrop amount
compared with spontaneous warming, although it significantly
shortened the duration of the afterdrop. Hotwater
bags and the charcoal heater then both provided
efficient and steady rewarming rates, whereas the rewarming
rate was small with chemical heating pads
and almost negligible with spontaneous warming.
Possible Mechanisms for the Findings
When the amount of heat accessible is limited such as
in a prehospital setting, external heat should be ap-
plied to the torso and areas with high surface heat
transfer (axillae, neck, and groin). 4−7,11,12,16 In a previous
torso-warming study, where different modalities
of forced-air warming were compared with a charcoal
heater and body-to-body rewarming, application
of heat to the torso effectively decreased afterdrop and
increased core rewarming. 16 This is likely due to the
close proximity of the heat source(s) to the heart and
lung circulation, and the fact that skin blood flow on
the torso is generally unaffected by temperature, unlike
the distal arms and legs. In this present study all
heat sources were therefore applied to the upper torso.
Although the evaluated heat sources were similar in
providing their heat content conductively to the skin
and underlying tissues, there were some important differences,
which affected warming effectiveness. Hotwater
bags and chemical heating pads, with their high
initial heat delivery to a relatively large surface area,
were both effective in attenuating afterdrop amount
and duration. The charcoal heater, with its similarly
high heat production but smaller surface area, had
less effect on afterdrop amount compared with spontaneous
warming, although it too significantly shortened
the duration of the afterdrop. Heat delivery from
the chemical heating pads then gradually declined
and, therefore, the subsequent core rewarming rate
was small. Hot-water bags and the charcoal heater,
on the other hand, provided high continuous heat delivery
and rendered effective rewarming rates. Conclusively,
high initial heat delivery to a large surface
area effectively decreased the afterdrop, whereas
consistent high heat delivery was required for core
rewarming.

Lundgren et al. FIELD TORSO-WARMING MODALITIES 377
Practical Implications
Several prehospital guidelines and review references
recommend active prehospital warming of cold patients,
especially if the patient is severely hypothermic
and endogenous shivering heat production is
inhibited. 1−7 Previous studies have shown that nonshivering
moderately to severely hypothermic patients
have a distinct thermal disadvantage because
their shivering heat production defense is abolished
and their basal metabolic rate is lower than normal,
thus the postcooling afterdrop will be large and
protracted. 14−16 In mildly hypothermic shivering patients,
exogenous skin heating attenuates shivering
heat production by an amount equivalent to the heat
donated. In these patients, the application of external
heat, although it might not decrease afterdrop
or increase the core rewarming rate, 8−13 might provide
other important advantages, including increased
comfort, decreased cardiac work, and preservation of
substrate availability. Accordingly, the application of
external heat might also be beneficial for initially normothermic
victims exposed to a cold environment.
In nonshivering hypothermic subjects, this study
demonstrated that chemical heating pads and hotwater
bags significantly decreased afterdrop at an
amount of about 0.6–0.7 ◦ C and that the charcoal heater
and hot-water bags significantly increased the rewarming
rate compared with spontaneous warming
by 0.5–0.6 ◦ C/h. All of the torso-warming modalities
also significantly decreased the time to Tes nadir and
the reversing of core cooling from about 88 minutes
with spontaneous warming to about 46–51 minutes
with active warming.
Previous retrospective analyses of trauma
registries 26,27 as well as prospective clinical
studies 28−30 have reported significant changes in
physiologic variables, such as increased oxygen consumption,
depletion of energy stores, disruption of
blood clotting mechanisms, increased fluid resuscitation
requirements, immune suppression, and development
of organ failure already at mild hypothermic
states compared with normothermic trauma victims.
Mild hypothermia is also demonstrated to increase
the risk of death in trauma patients, independent of
injury severity. 26,27,31 A significant correlation between
decreased duration of hypothermia by active core
rewarming and increased likelihood of successful
resuscitation and survival after trauma has also been
demonstrated. 28 Thus, diminishing or even reversing a
fall in core temperature, in an efficient yet safe manner,
is desirable in the field and during prehospital care
and transportation in order to improve the patient’s
condition upon admission to the emergency department.
Accordingly, although the clinical significance of
the differences in core temperature afterdrop or subsequent
rewarming rate measured in this study is not yet
fully known, we believe the impact of these prehospital
torso-warming modalities might be of great benefit
for an already compromised patient. To confirm the
clinical significance of our findings, we encourage
further prospective interventional clinical trials.
All of the evaluated torso-warming modalities are
portable, require no external power supply, and can
easily be used by laypersons, SAR personnel, or EMS
crew members without significant training. The charcoal
heater has a durable design yet is lightweight, is
easy to handle even under harsh conditions, and provides
heat over 8–12 hours using only one charcoal
fuel cell at high (250-W) settings. In protracted evacuation
and rescue operations, the charcoal heater therefore
has the advantage of sustained heat production.
Water bags are often available during backcountry
excursions or expeditions and do not take extra space
or effort to transport. Although it is certainly possible
and realistic for one person to reheat 4 liters of water
every 20 minutes as in this study, an external heat
source and significant effort are required for replenishing
the water bags. They would therefore be more appropriate
for scenarios in which the patient will remain
on the scene of the accident, waiting for evacuation.
Chemical heating pads are somewhat heavier and
take up more space than the other modalities but can
easily be transported in a vehicle such as in ground
or air ambulance units. Being effective in reversing the
initial fall in body core temperature, chemical heating
pads might prove valuable for initial thermal stabilization
of a cold victim. However, since the energy content
is limited, for continuous heat delivery we would
recommend that the chemical heating pads be replaced
about every 30 minutes.
Limitations
In order to limit the amount of meperidine necessary
to inhibit shivering, we had to try to expose the subjects
to the same relative cold stress depending on their
physical constitution, and, therefore, based on experiences
from prior pilot studies, immersion times differ
between the subjects. However, since each subject
served as his or her own control and immersion times
were exactly the same for each subject for all the warming
modalities, this should not have any impact on our
data.
CONCLUSIONS
In nonshivering hypothermic subjects, all warming
modalities significantly reduced the time to reversing
of core cooling. Greater sources of external heat,
such as chemical heating pads or hot-water bags, were
effective in attenuating the amount of core temperature
afterdrop, whereas sustained sources of external
heat, such as hot-water bags or the charcoal heater,

378 PREHOSPITAL EMERGENCY CARE JULY/SEPTEMBER 2009 VOLUME 13 / NUMBER 3
effectively established steady, efficient core rewarming.
Depending on scenario and available resources,
these promising results support the recommendation
to use charcoal heaters, hot-water bags, or chemical
heating pads as effective means for treating cold patients
in the field or during transport to definitive care.
References
1. Mills WJ. Field care of the hypothermic patient. Int J Sports Med.
1992;13:199–202.
2. Giesbrecht GG. Cold stress, near drowning and accidental hypothermia:
a review. Aviat Space Environ Med. 2000;71:733–52.
3. Giesbrecht GG. Emergency treatment of hypothermia. Emerg
Med. 2001;13:9–16.
4. Durrer B, Brugger H, Syme D. The medical on site treatment
of hypothermia. In: Elsensohn F (ed). Consensus Guidelines
on Mountain Emergency Medicine and Risk Reduction, 1st ed.
Italy: Casa Editrice Stefanoni ICAR MEDCOM, UIAA MED-
COM, 2001, pp 71–5.
5. Danzl DF. Accidental hypothermia. In: Auerbach PS (ed).
Wilderness Medicine, 4th ed. St. Louis, MO: Mosby, 2001, pp
135–77.
6. Hypothermia. Cold Injuries and Cold Water Near Drowning.
2nd revised ed. Stockholm, Sweden: National Board of Health
and Welfare, 2002.
7. State of Alaska Cold Injuries Guidelines. Juneau, AK: Department
of Health and Social Services, 2005. Available at: http://
www.chems.alaskagov/EMS/documents/AKColdinj2005.pdf.
Accessed May 12, 2009.
8. Giesbrecht GG, Bristow GK, Uin A, Ready AE, Jones RA. Effectiveness
of three field treatments for induced mild (33.0 ◦ C) hypothermia.
J Appl Physiol. 1987;63:2375–9.
9. Giesbrecht GG, Sessler DI, Mekjavic IB, Schroeder M, Bristow
GK. Treatment of mild immersion hypothermia by direct bodyto-body
contact. J Appl Physiol. 1994;76:2373–9.
10. Giesbrecht GG, Schroeder M, Bristow GK. Treatment of mild immersion
hypothermia by forced-air warming. Aviat Space Environ
Med. 1994;65:803–8.
11. Harnett RM, O’Brien EM, Sias FR, Pruitt JR. Initial treatment
of profound accidental hypothermia. Aviat Space Environ Med.
1980;51:680–7.
12. Collis ML, Steinman AM, Chaney RD. Accidental hypothermia:
an experimental study of practical rewarming methods. Aviat
Space Environ Med. 1977;48:625–32.
13. Sterba JA. Efficacy and safety of prehospital rewarming techniques
to treat accidental hypothermia. Ann Emerg Med.
1991;20:896–901.
14. Giesbrecht GG, Goheen MS, Johnston CE, Kenny GP, Bristow
GK, Hayward JS. Inhibition of shivering increases core temperature
afterdrop and attenuates rewarming in hypothermic humans.
J Appl Physiol. 1997;83:1630–4.
15. Goheen MS, Ducharme MB, Kenny GP, et al. Efficacy
of forced-air and inhalation rewarming using a human
model for severe hypothermia. J Appl Physiol. 1997;83:1635–
40.
16. Hultzer M, Xu X, Marrao C, Bristow GK, Chochinov A, Giesbrecht
GG. Pre-hospital torso-warming modalities for severe hypothermia:
a comparative study using a human model. Can J
Emerg Med. 2005;7:378–86.
17. Hamilton RS, Paton BC. The diagnosis and treatment of hypothermia
by mountain rescue teams: a survey. Wilderness Environ
Med. 1996;7:28–37.
18. Forgey WW (ed). Wilderness Medical Society Practice Guidelines
for Wilderness Emergency Care. Merillville, IN: ICS Books,
Inc., 1995.
19. Durnin JVGA, Womersley J. Body fat assessed from total density
and its estimation from skinfold thickness: measurements
on 481 men and women aged from 16 to 72 years. Br J Nutr.
1974;32(1):77–97.
20. Dubois D, Dubois E. A formula to estimate the approximate
surface area if height and weight are known. Arch Intern Med.
1916;17:863–71.
21. McArdle WD, Katch FI, Katch VL. Exercise Physiology, Energy,
Nutrition, and Human Performance. Philadelphia, PA: Williams
and Wilkins, 1996.
22. Cooper KE, Ferres HM, Kenyon JR, Wendt F. A comparison
of oesophageal, rectal and para-aortic temperatures during hypothermia
in man. Br J Surg. 1957;44:616–9.
23. Hayward JS, Eckerson JD, Kemna D. Thermal and cardiovascular
changes during three methods of resuscitation from mild hypothermia.
Resuscitation. 1984;11:21–33.
24. Layton RP, Mints WH Jr, Annis JF, Rack MJ, Webb P. Calorimetry
with heat flux transducers: comparison with a suit calorimeter. J
Appl Physiol. 1983;54:1361–7.
25. Henriques FC, Moritz AR. Studies of thermal injury: I. The conduction
of heat to and through skin and the temperatures attained
therein. A theoretical and an experimental investigation.
Am J Pathol. 1947;23:531–5.
26. Wang HE, Callaway CW, Peitzman AB, Tisherman SA. Admission
hypothermia and outcome after major trauma. Crit Care
Med. 2005;33:1296–301.
27. Shafi S, Elliott AC, Gentilello L. Is hypothermia simply a marker
of shock and injury severity or an independent risk factor for
mortality in trauma patients? Analysis of a large national trauma
registry. J Trauma. 2005;59:1081–5.
28. Gentilello LM, Jurkovich GJ, Stark MS, Hassantash SA, O’Keefe
GE. Is hypothermia in the victim of major trauma protective or
harmful? A randomized, prospective study. Ann Surg. 1997;226:
439-47; discussion 447–9.
29. WattsDD,TraskA,SoekenK,PerdueP,DolsS,KaufmannC.
Hypothermic coagulopathy in trauma: effect of varying levels
of hypothermia on enzyme speed, platelet function, and fibrinolytic
activity. J Trauma. 1998;44:846–854.
30. Watts DD, Roche M, Tricarico R, et al. The utility of traditional
prehospital interventions in maintaining thermostasis. Prehosp
Emerg Care. 1999;3:115–22.
31. Tisherman SA. Hypothermia and injury. Curr Opin Crit Care.
2004;10:512–9.

Lundgren et al. Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine 2011, 19:59
http://www.sjtrem.com/content/19/1/59
ORIGINAL RESEARCH Open Access
The effect of active warming in prehospital
trauma care during road and air ambulance
transportation - a clinical randomized trial
Peter Lundgren * , Otto Henriksson, Peter Naredi and Ulf Björnstig
Abstract
Background: Prevention and treatment of hypothermia by active warming in prehospital trauma care is
recommended but scientifical evidence of its effectiveness in a clinical setting is scarce. The objective of this study
was to evaluate the effect of additional active warming during road or air ambulance transportation of trauma
patients.
Methods: Patients were assigned to either passive warming with blankets or passive warming with blankets with
the addition of an active warming intervention using a large chemical heat pad applied to the upper torso. Ear
canal temperature, subjective sensation of cold discomfort and vital signs were monitored.
Results: Mean core temperatures increased from 35.1°C (95% CI; 34.7-35.5°C) to 36.0°C (95% CI; 35.7-36.3°C) (p <
0.05) in patients assigned to passive warming only (n = 22) and from 35.6°C (95% CI; 35.2-36.0°C) to 36.4°C (95% CI;
36.1-36.7°C) (p < 0.05) in patients assigned to additional active warming (n = 26) with no significant differences
between the groups. Cold discomfort decreased in 2/3 of patients assigned to passive warming only and in all
patients assigned to additional active warming, the difference in cold discomfort change being statistically
significant (p < 0.05). Patients assigned to additional active warming also presented a statistically significant
decrease in heart rate and respiratory frequency (p < 0.05).
Conclusions: In mildly hypothermic trauma patients, with preserved shivering capacity, adequate passive warming
is an effective treatment to establish a slow rewarming rate and to reduce cold discomfort during prehospital
transportation. However, the addition of active warming using a chemical heat pad applied to the torso will
significantly improve thermal comfort even further and might also reduce the cold induced stress response.
Trial Registration: ClinicalTrials.gov: NCT01400152
Keywords: hypothermia, body temperature regulation, thermal comfort, active warming, passive warming, prehospital
trauma care, emergency medical services (EMS)
Background
In a cold, wet or windy environment, an injured or ill person
is often exposed to a considerable cold stress. Heat
loss is often aggravated due to exhaustion, light, torn or
wet clothing, major bleeding, entrapment or the administration
of cold intravenous fluids or sedative drugs and
admission hypothermia is an independent risk factor associated
with worse outcome and higher mortality in trauma
patients [1-6]. The cold induced stress response will also
* Correspondence: peter.lundgren@surgery.umu.se
Division of Surgery, Department of Surgery and Perioperative Sciences,
Umeå University, Sweden
render great thermal discomfort which might increase the
experience of pain and anxiety, even in still normothermic
patients [7]. Thus, in addition to immediate care for life
threatening conditions, actions to reduce cold exposure
and prevent further heat loss is an important and integrated
part of prehospital primary care. Initial measures
should be taken to get the patient into shelter, remove wet
clothing and insulate the patient from ambient weather
conditions and ground chill within adequate wind- and
waterproof insulation ensembles (passive warming). In
addition, depending on the victim’s physiological status,
body core temperature, available resources and expected
© 2011 Lundgren et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.

Lundgren et al. Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine 2011, 19:59
http://www.sjtrem.com/content/19/1/59
duration of evacuation, the application of external heat
(active warming) is in most guidelines recommended to be
considered to aid in protection from further cooling during
evacuation and transport to definitive care [8-12].
Several studies on mildly hypothermic (body core temperature,
Tco = 32-35°C) shivering subjects have found
that exogenous skin heating attenuates shivering heat
production by an amount equivalent to the heat donated
[13-15]. Thus, in a mildly hypothermic shivering victim,
external warming generally does not decrease afterdrop
or increase rewarming rate, however it might provide
other advantages including increased comfort, decreased
cardiac work and preserved substrate availability. When
shivering is diminished or absent, as in moderate (Tco =
28-32°C) to severe (T co < 28°C) hypothermia or otherwise
impaired due to the overall medical condition of the
patient (i.e. old age, alcohol or drug ingestion, head or
spinal injury, severe trauma or depleted metabolic energy
substrates) some form of exogenous external or internal
heat is required, otherwise afterdrop will continue and
little or no rewarming will occur [16,17].
Accordingly, effective prehospital field treatment of
patients exposed to cold stress is considered of utmost
importance to improve the medical condition on admission
to the emergency room and active warming already
in the field is considered one important part of such
treatment. Since the warming modalities need to be portable
and easily handled by Search and Rescue (SAR) or
Emergency Medical Services (EMS) personnel there are
limited treatment options in the field or during transport
to definitive care. Chemical heat pads, hot water bottles,
plumbed water filled blankets, charcoal fueled heat pacs,
forced air warming and resistive heating devices are commonly
used and advised [8-12], but the lack of studies in
field conditions is noticed [18] and to the authors’ knowledge,
only two randomized clinical trials have evaluated
the effectiveness of such modalities in the field [19,20].
We therefore decided to evaluate the effect of an active
warming intervention on cold stressed trauma patients
using chemical heat pads, previously evaluated in a laboratory
study [17], as one possible field applicable warming
device during road or air ambulance transportation of
trauma patients. Primary outcome measures were body
core temperature, cold discomfort and vital signs.
Methods
Design and settings
The study was designed as a randomized, clinical trial of
prehospital active warming intervention for trauma
patients, where enrolled patients were assigned to either
passive warming with blankets (routine care) or passive
warming with blankets with the addition of an active
warming intervention using a large chemical heat pad
applied to the upper torso. Ethical approval was obtained
Page 2 of 7
from the Regional Ethical Review Board at Umeå University.
The study was conducted from December 2007 until
May 2010. Fourteen road ambulance units and one helicopter
unit, serving a primarily suburban area in the
northern parts of Sweden with about 125 000 inhabitants,
were selected for the study. After given both written and
verbal instructions, the participating EMS personnel carried
out the study as a part of their normal duty, without
interference by the investigators.
Population
Subjects were sequential trauma patients, age ≥ 18 years,
who had sustained an injury outdoors and were transported
by one of the participating EMS units. Patients
were excluded if initial level of consciousness was affected,
(Glasgow Coma Scale < 15), or if duration of transportation
was expected to be shorter than 10 minutes. As the
aim of the study was to investigate the effect of active
warming intervention in cold stressed patients, those
patients who had already received active warming or had
been taken indoors for more than 10 minutes before EMS
unit arrival or had an initial cold discomfort rating ≤ 2
were also excluded.
Protocol
At the scene of injury event, all patients initially received
routine trauma care, including passive warming with
blankets. After loading into the ambulance or helicopter,
informed consent to be part of the study was obtained.
Enrolled patients then were selected for either passive
warming or passive warming with the addition of active
warming by opening of sequentially numbered and sealed
envelopes containing randomized study protocols. A
tympanic sensor was placed in the patient’s ear canal and
the outer ear sealed with a soft insulation cover. After
5 minutes an initial recording of ear canal temperature,
cold discomfort, heart rate, blood pressure and respiratory
rate, was obtained before active warming was begun
if assigned. Apart from air temperature set to 25°C in the
transportation unit, no other regulations were appointed.
The number of blankets applied and specific care, such
as immobilization or intravenous fluids and medications
were provided according to standard trauma protocols.
Repeated recordings of ear canal temperature, cold discomfort
and vital signs were obtained every 30 minutes
and upon arrival to the receiving hospital or health care
center.
Passive warming
The participating ambulance units all had polyester blankets
(200 × 135 × 0.4 cm, 1.200 g, 2.4 clo), woollen blankets
(190 × 135 × 0.5 cm, 1.900 g, 2.7 clo) and one rescue
blanket (nylon outer with synthetic filling and cotton
inner, 275 × 125 × 0.7 cm, 2.300 g, 3.6 clo) as part of

Lundgren et al. Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine 2011, 19:59
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their standard equipment. The type and number of blankets
applied in each case were selected according to the
EMS crew judgement without any regulations by the
investigators. For comparative reasons the polyester blanket
was accounted for as 1.0 blanket whereas the woollen
blanket was accounted for as 1.1 blankets and the rescue
blanket was accounted for as 1.5 blankets depending on
their thermal insulation value (clo) determined according
to European Standard for assessing requirements of
sleeping bags [21].
Active warming intervention
A chemical heat pad (Dorcas AB, Skattkärr, Sweden), was
selected as the active warming device. In a previous
laboratory study this chemical heat pad, applied both to
the anterior and posterior upper torso, was appreciated for
its effectiveness in transferring heat to a cold person [17].
To simplify for the EMS crew, in this study the chemical
heat pad on the posterior upper torso was left out. After
activation, the heat pad (42 × 25 × 2 cm), reaching about
50°C within 2 minutes, was applied across the anterior
upper torso, leaving only one layer of thin clothing
between the heat pad and the skin. If the clothing had to
be removed to gain necessary access to the patient, the
heat pad was placed in an ordinary pillow-case to prevent
burns to the skin. Following the initial chemical reaction,
the surface temperature of the heat pad gradually declines
[17]. To maintain effective heat transfer during longer
transportations, the heat pad was thus replaced every 30
minutes.
Monitoring
A closed ear canal temperature sensor (Smiths Medical,
Ltd., UK) was selected to monitor core temperature
changes (± 0.2°C) during transportation. Ear canal temperature
has been shown to correlate well with oesophageal
temperature [22,23]. If properly sealed from the
ambient air, closed ear canal temperature is also reliable in
subzero and wind conditions [22] and thus considered the
most accurate non invasive method of measuring body
core temperature in the field [10-12]. After visual inspection
of the outer ear to rule out any injuries, the sensor
was gently placed in the middle of the ear canal. In addition
to the outer soft cell foam cylinder that conforms to
the ear canal and seals out ambient air, a soft insulation
cover was placed on the outer ear and secured with Velcro
around the head. The ear canal sensor was then connected
to a temperature monitor (Novamed, Inc., USA) and left
in place during the whole transportation.
Cold discomfort was monitored using a numerical rating
scale [24], whereby the subjects estimated their sensation
of cold to the whole body, not specific body
parts, providing values from 0 to 10, where 0 indicated
Page 3 of 7
no sensation of cold and 10 indicated unbearable sensation
of cold.
Vital signs were monitored using routine equipment and
data collection sheets were filled out during transportation
by the EMS personnel. In addition to ear canal temperature,
vital signs, cold discomfort and overall satisfaction of
care, the following information was recorded: time from
injury to EMS unit arrival, on-scene duration, transportation
time, outdoor temperature, wind speed, ambulance
unit indoor temperature, patient characteristics, clothing
characteristics, the type and number of blankets applied,
immobilization and the administration of warm intravenous
fluids and medications.
Data analysis
According to pre-study power calculations, with an estimated
difference in core temperature of ≥ 0.5°C or cold
discomfort rating of ≥ 2, an alpha of 0.05 and a power of
0.90, the minimum number of patients required to
achieve statistical significance was 21 in each group and
the study was ended after, with some margin, a sufficient
number of patients had successfully been enrolled.
Groups were compared using Mann-Whitney U-test for
interval and ordinal data and Chi-2 or Fisher’s exact test
for nominal data, whereas pair wise related variable comparisons
was made using the Wilcoxon Signed-Rank test.
In addition, change in cold discomfort rating was characterized
as increased, unchanged or decreased and the difference
between groups was analyzed using Fisher’s exact
test. Statistical significance was defined as p < 0.05.
Results
Patient characteristics
Fifty-one trauma patients were enrolled in the study. Of
these, one patient wished to end the study prior to arrival
to the receiving hospital and two were excluded because
of breach of protocol (assigned intervention was not
given). Thus, a total of 48 patients, all subjected to blunt
trauma, with a mean coded Revised Trauma Score (RTS)
[25] of 7.83 (range 7.55 - 7.84), successfully completed the
study, being randomized to either passive warming with
blankets (n = 22) or passive warming with blankets with
the addition of active warming (n = 26). The included
patients were 19 male and 29 female and there were no
significant differences between the two groups on morphometric
or demographic characteristics (table 1).
Environment
The average ambient air temperature at the scene of accident
was -4 ± 7°C (mean ± SD) and the average time
from the injury until the patient was loaded into the EMS
unit (cold exposure) was 73 ± 53 minutes with no significant
differences between the two groups. The mean

Lundgren et al. Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine 2011, 19:59
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Table 1 Patient characteristics and confounding factors
interior unit temperature during transport was 20 ± 3°C
and the mean number of blankets applied was 2.5 ± 1.1
with no significant differences between the two groups.
There were also no significant differences between the
two groups in distribution of clothing thickness or wetness,
the extent of undressing, the incidence of whole
body fixation, the amount of intravenous fluids transfused
or the incidence of intravenous opioids or sedatives
administered during transport (table 1).
Primary outcome
The average transportation time to the receiving hospital
or health care centre was 35 ± 26 minutes (mean ± SD)
with no significant differences between the two groups.
Thus, at the second measurement, performed at an average
of 26 ± 7 minutes all 48 subjects were included,
whereas at the third measurement, performed at an averageof58±5minutesonly12subjectsremained.The
analysis of primary outcome variables was therefore terminated
after the second measurement.
Mean initial ear canal temperature was 35.1°C (95% CI;
34.7 - 35.5°C) in patients assigned to passive warming
only and 35.6°C (95% CI; 35.2 - 36.0°C) in those assigned
to additional active warming with no significant differences
between the two groups. At the second measurement,
mean ear canal temperatures in both groups were
significantly increased to 36.0°C (95% CI; 35.7 - 36.3°C)
Passive warming
(n = 22)
Active warming
(n = 26)
Page 4 of 7
Patient characteristics
Gender (male/female) 9/13 10/16
Age (years) 45 (34 - 55) 43 (36 - 50)
Body Mass Index
Environment
25.0 (22.8 - 27.3) 25.4 (23.6 - 27.3)
Outdoor temperature (°C) -6 (-9 - -2) -3 (-6 - -1)
Outdoor wind speed (m/s) 2 (1 - 3) 2 (1 - 4)
Interior unit temperature (°C) 20 (19 - 21) 20 (19 - 21)
Cold exposure (min) 64 (41 - 88) 81 (61 - 101)
Time to 2 nd measurement (min) 24 (21 - 28) 27 (24 - 29)
Total transportation (min) 33 (25 - 41) 37 (25 - 49)
Clothing (light/medium/heavy) 5/6/9 4/9/13
Clothing (dry/moist/wet)
Treatment during transport
13/2/4 21/3/1
No. of blankets 2.7 (2.2 - 3.2) 2.3 (1.9 - 2.7)
Undress (none/partial/total) 12/8/2 14/10/1
Whole body fixation (yes/no) 8/13 8/17
Intravenous fluids (ml) 91 (31 - 151) 50 (0 - 107)
Intravenous opioids (yes/no) 10/12 14/12
Intravenous sedatives (yes/no) 2/20 5/21
Values are mean (95% confidence interval) or number of patients. The internal drop-out of any variable was ≤ 3 patients and there are no significant differences
between groups (p < 0.05).
and 36.4°C (95% CI; 36.1 - 36.7°C) respectively with no
significant differences between the two groups (table 2).
The initial median cold discomfort rating in patients
assigned to passive warming only was 5 (IQR; 4 - 7) and
the initial median cold discomfort rating in patients
assigned to passive warming with the addition of active
warming was 7 (IQR; 5-8) with no significant differences
between the two groups. At the second measurement,
cold discomfort was significantly reduced in both groups.
However, in the group assigned to passive warming only,
15 out of 22 patients presented a decrease in cold discomfort,
whereas in the group assigned to additional
active warming all 26 patients presented a decrease in
cold discomfort ratings, the difference in cold discomfort
change being statistically significant (table 2).
There were no statistically significant differences in
initial vital signs between the two groups. At the second
measurement, the vital signs were statistically unchanged
for the patients assigned to passive warming only, whereas
patients assigned additional active warming presented a
small but statistically significant reduction in mean heart
rate and respiratory frequency (table 2).
Discussion
Overview
This study evaluates the effectiveness of active warming
in prehospital trauma care using a large chemical heat

Lundgren et al. Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine 2011, 19:59
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Table 2 Primary outcome
Passive warming
(n = 22)
Active warming
(n = 26)
Body core temperature (°C) *
1 st measurement 35.1 (34.7 - 35.5) 35.6 (35.2 - 36.0)
2 nd measurement
Cold discomfort **
36.0 (35.7 - 36.3) † 36.4 (36.1 - 36.7) †
1 st measurement 5 (4 - 7) 7 (5 - 8)
2 nd measurement 3 (0 - 5) † 2(1-3)†
↳ increased 1 0
↳ unchanged 5 0
↳ decreased
Vital signs *
Heart rate
15 26 ‡
1 st measurement 83 (77 - 90) 84 (78 - 90)
2 nd measurement
Systolic blood pressure
82 (76 - 87) 80 (75 - 86) †
1 st measurement 138 (129 - 147) 136 (127 - 145)
2 nd measurement
Respiratory rate
134 (124 - 143) 131 (124 - 139)
1 st measurement 17 (16 - 19) 18 (16 - 20)
2 nd measurement 17 (15 - 18) 16 (14 - 18) †
Revised Trauma Score 7.84 (7.84 - 7.84) 7.83 (7.80 - 7.84)
Values are * mean (95% confidence interval) or ** median (interquartile range)
and number of patients. The internal drop-out of any variable was ≤ 2 patients.
† Significant difference within the same group (Mann-Whitney U-test, p < 0.05)
‡ Significant difference between groups (Fisher’s exact test, p < 0.05)
pad applied to the upper torso in addition to passive
warming with blankets during transportation to definitive
care. Over the first 30 minutes of prehospital transportation,
both patients receiving passive warming only
and patients receiving passive warming with the addition
of active warming presented a statistically significant
increase in body core temperature as well as improved
cold discomfort. However, in the group assigned to passive
warming only, 2/3 of the patients presented a
decrease in cold discomfort, whereas all patients in the
group assigned to additional active warming presented a
decrease in cold discomfort ratings, the difference in
cold discomfort change being statistically significant.
Possible mechanism for findings
In previous laboratory studies on mildly hypothermic shivering
subjects, exogenous skin heating has been shown
to attenuate shivering heat production by an amount
equivalent to the heat donated [13-15]. Accordingly, in
this study, enrolling trauma patients with an initial body
core temperature of about 35°C and preserved shivering
capacity, active warming had no additional effect on body
core temperature compared to passive warming only. In
contrast, two previous randomized clinical trials found a
decrease in body core temperature with passive warming
only, whereas with additional active warming using either
Page 5 of 7
electrically heated blankets [19] or multiple chemical
heat pads [20], body core temperature was increased during
transportation. Since passive warming only as an adequate
treatment alternative presupposes intact shivering
capacity and enough insulation in relation to cold stress
and ambient environmental conditions, differences
regarding these factors might explain differences between
studies.
Although body core temperature was increased, only 2/3
of the patients assigned to passive warming only presented
a decrease in cold discomfort whereas all patients assigned
to additional active warming presented a decrease in cold
discomfort during transportation. This beneficial effect on
thermal comfort by application of a chemical heat pad to
the upper torso is probably explained by a combination of
reduction in shivering thermogenesis and increased skin
temperature. Although shivering was not monitored per
se in this study, a reduction of the cold induced stress
response was indicated by a small but statistically significant
decrease in respiratory frequency and heart rate in
patients assigned to active warming, whereas patients
assigned to passive warming presented no significant
change in these parameters during transportation.
Practical implications
Admission hypothermia is an independent risk factor associated
with worse outcome in trauma patients and previous
retrospective analysis of trauma registries as well as
prospective clinical studies have reported significant
changes in physiologic variables, such as increased oxygen
consumption, depletion of energy stores, disruption of
blood clotting mechanisms, increased fluid resuscitation
requirements, immune suppression and development of
organ failure already at mild hypothermic states compared
to normothermic trauma patients [1-6].
Owing to peripheral vasoconstriction, the temperature
in the periphery of the body starts to decline long before
body core temperature is affected. After removal from the
cold environment there is a temperature equalisation
between the warm body core and the cold peripheral parts
contributing to a continuous fall in body core temperature,
designated the afterdrop phenomenon. The magnitude of
the afterdrop, which can be considerable and amount to
several degrees, is dependent on temperature gradients in
the tissues, peripheral circulation and endogenous heat
production. Thus, initial measures in prehospital care of
cold stressed patients are aiming at avoiding further heat
loss to the environment and reducing the amount and
duration of the afterdrop [8-12].
According to this study on cold stressed trauma
patients with an initial body core temperature of about
35°C and preserved shivering capacity, passive warming,
if adequate, is an effective treatment to prevent afterdrop,
establish a steady rewarming rate and reduce cold

Lundgren et al. Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine 2011, 19:59
http://www.sjtrem.com/content/19/1/59
discomfort during transportation to definitive care. However,
additional active warming had a beneficial effect in
improving thermal comfort and indicated a small reduction
of the cold induced stress response. Even in these
mild hypothermic states, active warming might be of
considerable clinical importance, especially in scenarios
with diminished to absent shivering or inadequate passive
warming. In a sustained cold outdoor environment, such
as in prolonged extrications or in multiple casualty situations
where available insulation often is inadequate, shivering
will then be maintained in order to prevent
afterdrop, thereby increasing respiratory and circulatory
demands which might be detrimental for an already compromised
patient. The application of external heat would
therefore be even more important to reduce shivering
strain. Also, if shivering is diminished or absent due to
moderate or severe hypothermia or due to the patient’s
overall medical condition some form of exogenous heat
is most likely required, otherwise afterdrop will continue
and little or no rewarming will occur [16,17]. Improved
thermal comfort might also relieve the experience of pain
and anxiety and contribute to the physiological wellbeing
of the patient during prehospital care.
Limitations
In addition to body core temperature, subjective sensation
of cold discomfort and vital signs, other parameters
such as oxygen consumption (as a measure of shivering)
and skin temperature would have been important and
useful supplements as indicators of cold stress.
Further research
The thermal effectiveness of active warming in prehospital
trauma care has only been evaluated in a few previous
clinical trials [19,20] and the results are diverging. Various
degrees of injuries as well as different warming modalities
and different amounts of passive warming might explain
differences between the studies. All studies are also relatively
small and included patients suffering from not more
than mild hypothermia. Thus, thermal effectiveness of
active warming in prehospital trauma care deserves further
research, especially including more severely injured
patients suffering from moderate or severe hypothermia.
Conclusion
In mildly hypothermic trauma patients, with preserved
shivering capacity, adequate passive warming is an effective
treatment to establish a slow rewarming rate and to
reduce cold discomfort during prehospital transportation.
However, the addition of active warming using a
chemical heat pad applied to the torso will significantly
improve thermal comfort even further and might also
reduce the cold induced stress response.
Acknowledgements and Funding
The study was supported by the National Board of Health and Welfare,
Sweden.
Authors’ contributions
The authors contibuted in the following way to the paper:
PL: Design of the study, aquisition of data, analysis and interpretation of
data and writing of the manuscript.
OH: Design of the study, aquisition of data, analysis and interpretation of
data and writing of the manuscript.
PN: Interpretation of data and critically revising the manuscript.
UB: Design of the study, interpretation of data and critically revising the
manuscript.
All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 21 July 2011 Accepted: 21 October 2011
Published: 21 October 2011
Page 6 of 7
References
1. Ireland S, Endacott R, Cameron P, Fitzgerald M, Paul E: The incidence and
significance of accidental hypothermia in major trauma - A prospective
observational study. Resuscitation 2011, 82(3):300-306.
2. Beilman GJ, Blondet JJ, Nelson TR, Nathens AB, Moore FA, Rhee P,
Puyana JC, Moore EE, Cohn SM: Early hypothermia in severly injured
trauma patients is a significant risk factor for multiple organ dysfunction
syndrome but not mortality. Ann Surg 2009, 249(5):845-50.
3. Martin RS, Kilgo PD, Miller PR, Hoth JJ, Meredith JW, Chang MC: Injury
associated hypothermia: an analysis of the 2004 National Trauma Data
Bank. Shock 2005, 24(2):114-8.
4. Wang HE, Callaway CW, Peitzman AB, Tisherman SA: Admission
hypothermia and outcome after major trauma. Critical Care Medicine
2005, 33(6):1296-301.
5. Shafi S, Elliott AC, Gentilello L: Is hypothermia simply a marker of shock
and injury severity or an independent risk factor for mortality in trauma
patients? Analysis of a large national trauma registry. Journal of Trauma-
Injury Infection & Critical Care 2005, 59(5):1081-5.
6. Gentilello LM, Jurkovich GJ, Stark MS, Hassantash SA, O’Keefe GE: Is
hypothermia in the victim of major trauma protective or harmful? A
randomized, prospective study. Annals of Surgery 1997, 226(4):439-47,
discussion 447-9.
7. Robinson S, Benton G: Warmed blankets: an intervention to promote
comfort to elderly hospitalized patients. Geriatric nursing 2002,
23:320-323.
8. Danzl DF: Accidental Hypothermia. In Wilderness medicine.. 5 edition.
Edited by: Auerbach P. Philadelphia. PA: Mosby Elsevier; 2007:125-159.
9. Giesbrecht GG, editor: Hypothermia frostbite and other cold injuries
prevention, survival, rescue and treatment. Seattle, WA: Mountaineers
Books;, 2 2006.
10. Socialstyrelsen: Hypothermia: cold injuries and cold water near drowning.
Stockholm, Sverige: The National Board of Health and Welfare
(Socialstyrelsen);, 2 2002.
11. Durrer B, Brugger H, Syme D: The medical on site treatment of
hypothermia. In Consensus Guidelines on Mountain Emergency Medicine and
Risk Reduction.. 1 edition. Edited by: Elsensohn F. Italy: Casa editrice
stefanoni - lecco; 2001:71-75.
12. State of Alaska cold injuries guidelines: Alaska Emergency Medical
Services Program, Department of Health and Social Services, Juneau.
[http://www.chems.alaska.gov/EMS/documents/AKColdInj2005.pdf],
Accessed 19 April 2011.
13. Giesbrecht GG, Bristow GK, Uin A, Ready AE, Jones RA: Effectiveness of
three field treatments for induced mild (33.0°c) hypothermia. J Appl
Physiol 1987, 63:2375-79.
14. Sterba JA: Efficacy and safety of prehospital rewarming techniques to
treat accidental hypothermia. Ann Emerg Med 1991, 20:896-901.
15. Giesbrecht GG, Sessler DI, Mekjavic IB, Schroeder M, Bristow GK: Treatment
of mild immersion hypothermia by direct body-to-body contact. J Appl
Physiol 1994, 76:2373-9.

Lundgren et al. Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine 2011, 19:59
http://www.sjtrem.com/content/19/1/59
16. Giesbrecht GG, Goheen MS, Johnston CE, Kenny GP, Bristow GK,
Hayward JS: Inhibition of shivering increases core temperature afterdrop
and attenuates rewarming in hypothermic humans. J Appl Physiol 1997,
83:1630-4.
17. Lundgren JP, Henriksson O, Pretorius T, Cahill F, Bristow G, Chochinov A,
Pretorius A, Bjornstig U, Giesbrecht GG: Field Torso Warming Modalities: A
Comparative Study Using a Human Model. Prehosp Emerg Care 2009,
3:371-378.
18. Allen PB, Salyer SW, Dubick MA, Holcomb JB, Blackbourne LH: Preventing
hypothermia: comparison of current devices used by the US Army in an
in vitro warmed fluid model. J Trauma 2010, 69(Suppl 1):S154-61.
19. Kober A: Effectiveness of resistive heating compared with passive
warming in treating hypothermia associated with minor trauma: a
randomized trial. Mayo Clin Proc 2001, 76:369-375.
20. Watts DD: The utility of traditional prehospital interventions in
maintaining thermostasis. Prehosp Emerg Care 1999, 3:115-122.
21. EN 13537: Requirements for sleeping bags. Brussels: European Committee
for Standardization; 2002.
22. Walpoth BH, Galdikas J, Leupi F, Muehlemann W, Schlaepfer P, Althaus U:
Assesment of hypothermia with a new “tympanic” thermometer. J Clin
Monit 1994, 10:91-96.
23. Webb GE: Comparison of esophageal and tympanic temperature
monitoring during cardiopulmonary bypass. Anesth Analg 1973,
52(5):729-33.
24. Parsons KC: Thermal comfort. In Human thermal environments: the effects of
hot, moderate, and cold environments on human health, comfort and
performance.. 2 edition. Edited by: Parsons KC. London, UK: Taylor
2003:196-228.
25. Champion HR, Sacco WJ, Copes WS, Gann DS, Gennarelli TA, Flanagan ME:
A revision of the trauma score. J Trauma 1989, 29(5):623-9.
doi:10.1186/1757-7241-19-59
Cite this article as: Lundgren et al.: The effect of active warming in
prehospital trauma care during road and air ambulance transportation -
a clinical randomized trial. Scandinavian Journal of Trauma, Resuscitation
and Emergency Medicine 2011 19:59.
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Page 7 of 7

III

The Effect of Active Warming on Cold Discomfort in Field Treatment of
Trauma Patients – a Clinical Randomized Trial
Peter Lundgren MD; Otto Henriksson MD; Peter Naredi MD PhD; Ulf Björnstig MD PhD
Division of Surgery, Department of Surgery and Perioperative Sciences,
Umeå University, Sweden
Running head: Active External Warming in Field Trauma Care
Corresponding author: Dr. Peter Lundgren
Division of Surgery,
Department of Surgery and Perioperative Sciences
SE-90185 Umeå, Sweden
E-mail: peter.lundgren@surgery.umu.se
Telephone: +46706678316
Fax: +4690771755
E-mail for all authors: peter.lundgren@surgery.umu.se
Conflicts of Interest: None declared
otto.henriksson@surgery.umu.se
peter.naredi@surgery.umu.se
ulf.bjornstig@surgery.umu.se
Source of Funding: The National Board of Health and Welfare, Sweden
Key words: hypothermia, prehospital trauma care, field treatment, emergency medical
services (EMS), active warming, passive warming, thermal comfort
1

Abstract
Background: Adequate field treatment to reduce cold exposure is considered of vitally
important to improve the medical condition of the trauma patientn upon admission to hospital,
and the application of active warming in the field is considered an important part of such
treatment.
Objective: The objective of this study was to evaluate the effect of active warming
intervention on cold discomfort during rescue and transport of patients with skiing injuries in
a cold outdoor environment.
Methods: Patients were assigned to either passive warming alone, or passive warming with
the addition of active warming, using a large chemical heat pad applied to the anterior upper
torso. Subjective sensation of cold discomfort was monitored at the scene of injury event and
upon arrival to the recieving first aid center or EMS unit.
Results: In patients assigned to passive warming alone (n = 9), initial median cold
discomfort, 5 (IQR; 2 – 5), remained at the same level, 5 (IQR; 3 – 6), during rescue and
transport, whereas in patients assigned to additional active warming (n = 11) initial median
cold discomfort, 4 (IQR; 3 – 7), decreased significantly, 1 (IQR; 0 – 3), (p < 0.05).
Conclusions: Additional active warming using a chemical heat pad applied to the anterior
upper torso significantly improved thermal comfort during field treatment and transport of
cold stressed trauma patients in a cold outdoor environment.
2

Introduction
In a prehospital rescue scenario an injured or ill person is often subject to a considerable cold
stress, especially in remote areas, where the search for and where the evacuation of the victim
might be prolonged and also where the first assessment and treatment take place outdoors.
Harsh weather conditions, insufficient or wet clothing, immobilization, contact with cold
surfaces, significant blood loss, and the administration of cold intravenous fluids or sedative
drugs might all aggravate cold stress and contribute to a subsequent decrease in body core
temperature (1). Admission hypothermia, defined as a body core temperature below 35°C, is
an independent risk factor associated with worse outcome in trauma patients (2 - 4) The cold
induced stress response causes great thermal discomfort which might increase pain and
anxiety, even in normothermic patients (5).
Actions to reduce cold exposure and prevent further heat loss are important and integrated
parts of prehospital primary care. Shelter and adequate wind- and waterproof insulation
ensembles (passive warming) are imperative. In addition, the application of external heat
(active warming) is recommended to protect from further cooling, during evacuation and
transport to definitive care. Active warming has the benefit of attenuating cold induced
shivering and thereby decreases cardiac and respiratory demands and preserves substrate
availability. When shivering is absent or diminished some form of external heat is required,
otherwise cooling will continue and little and no core rewarming will occur. (1, 6 - 9)
Since the warming modalities need to be portable and easily handled, treatment options the
field are limited. Chemical heat pads, hot water bottles, plumbed water filled blankets,
charcoal fueled heat packs, electrical heating pads and blankets are commonly used or advised
(1, 6, 7), but to the authors’ knowledge, only two previous randomized clinical studies have
evaluated the effectiveness of such modalities in the field (10, 11).
In a recent study we evaluated the effect of prehospital active warming during road or air
ambulance transportation of cold stressed trauma patients with preserved shivering capacity
(12). The addition of a large chemical heat pad applied to the upper torso significantly
improved thermal comfort compared to passive warming alone, but had no additional effect
on core warming. As this study was conducted during transport in a heated environment we
decided to address another part of the prehospital setting, evaluating the effect of the same
active warming device regarding sensation of cold discomfort, when applied at the scene of
accident and during transport in a cold outdoor environment.
3

Methods
The study was designed as a randomized clinical trial of prehospital active warming
intervention on trauma patients with skiing injuries. Patients were assigned to either passive
warming alone (routine care) or passive warming with the addition of active warming by a
large chemical heat pad applied to the anterior upper torso. Ski patrol units at three minor ski
resorts in the northern parts of Sweden were selected for the study, which was conducted
during two consecutive winter seasons. After being given both written and verbal instructions,
the participating ski patrol personnel carried out the study as a part of their normal duty,
without interference by the investigators. Ethical approval was obtained from the Regional
Ethical Review Board in Umeå..
Subjects were sequential patients, age 18 years, who had sustained an injury on the ski
slopes and were attended and transported by one of the participating ski patrols. Patients were
excluded if initial level of consciousness was affected (Glasgow Coma Scale

they are hard to separate in a practical situation. After unloading the patient the ski patrol
personnel filled out the data collection sheets. In addition to cold discomfort, the time of
injury event, on-scene duration, transport time, outdoor temperature and wind speed, patient
characteristics, clothing characteristics, and number of blankets applied was recorded.
Groups were compared using the Mann-Whitney U-test for nonparametric continuous data
and Chi-square test for nominal data, whereas pair-wise related variable comparisons was
made using the Wilcoxon Signed Rank test. In addition, change in cold discomfort was
characterized as increased, unchanged or decreased and the difference between groups was
analyzed using Fisher’s exact test. Statistical significance was defined as p < 0.05 (two-sided).
Results
A total of twenty patients were enrolled in the study, and they were randomized to either
passive warming (n = 9) or passive warming with the addition of active warming (n = 11).
There were no significant differences between the two groups regarding patient characteristics
or environmental factors (table 1). The mean ambient air temperature at the scene of accident
was -6 ± 4 °C (mean ± SD) and the mean wind velocity was 4 ± 3 m/s. The average time from
the injury until ski patrol arrival and first cold discomfort rating was 13 ± 9 minutes followed
by an average treatment and transport time to the receiving first aid center or EMS unit of 23
± 10 minutes. There were also no differences in distribution of clothing thickness or moisture
or the number of blankets applied between the two groups.(Table 1).
At the scene of injury, the initial median cold discomfort was 5 (IQR; 2 – 5) in patients
assigned to passive warming and 4 (IQR; 3 – 8) in patients assigned to additional active
warming with no significant differences between the two groups. When patients were
unloaded at the receiving first aid center or EMS unit, cold discomfort was significantly
reduced to 1 (IQR; 0 – 3) in patients receiving additional active warming but remained at 5
(IQR; 3 – 6) in patients receiving passive warming alone (Fig 1). In addition when change in
cold discomfort was characterized as increased, unchanged or decreased, 9 of 11 patients
assigned to additional active warming presented a decrease in cold discomfort and two
remained at their initial cold discomfort, whereas only 3 of 9 patients assigned to passive
warming alone presented a decrease in cold discomfort, one remained at initial cold
discomfort and 5 presented an increase in cold discomfort. The difference between groups in
cold discomfort change was statistically significant.
5

Discussion
This study evaluated the effect on cold discomfort of active field warming of trauma patients
injured on the ski slopes, where patients were assigned to either passive warming alone
(routine care) or passive warming with the addition of active warming by a large chemical
heat pad applied to the anterior upper torso. During rescue and transport, patients receiving
additional active warming experienced statistically significantly improved thermal comfort,
whereas patients receiving passive warming alone experienced sustained cold discomfort.
Reducing cold exposure is considered of utmost importance to prevent further cooling during
rescue and transport to definitive care, and the application of external heat in the field is
considered an important part of prehospital treatment (1, 6-9). This study demonstrated that
additional active field warming rendered statistically significantly improved thermal comfort
compared to passive warming alone during treatment and transport in a cold environment.
Improved thermal comfort might have the potential of relieving psychological stress such as
the experience of pain and anxiety (5), which is an important but easily forgotten part of
medical care. Such psycological stress might also comprise a considerable physiological
stress to the patient by increasing respiratory and cardiac work. The combined physiological
and psychological benefits of active field warming might therefore be of great importance.
In order not to intervene with the work of the ski patrol units, we did not measure body core
temperature, skin temperature, vital signs or oxygen consumption, which together with a
small study population are obvious limitations of this study.
The thermal effectiveness of active external warming in prehospital trauma care has only been
evaluated in a few previous clinical trials (10 - 12) and the results are diverging. Various
degrees of injuries, as well as different warming modalities and different amounts of passive
warming, might explain differences between the studies. All studies are also relatively small
and included patients suffering from not more than mild hypothermia. Thus, thermal
effectiveness of active warming in prehospital trauma care deserves further research,
especially including more severely injured patients suffering from moderate or severe
hypothermia.
Conclusion
Additional active external warming using a chemical heat pad applied to the anterior upper
torso significantly improved thermal comfort during field treatment and transport of cold
stressed trauma patients in a cold outdoor environment.
6

Acknowledgements
The study was supported by the National Board of Health and Welfare, Sweden.
7

References
1. Danzl DF. Accidental Hypothermia. In: Auerbach P, editor. Wilderness medicine. 5 th ed.
Philadelphia. PA: Mosby Elsevier; 2007, pp 125-159.
2. Ireland S, Endacott R, Cameron P, Fitzgerald M, Paul E. The incidence and significance
of accidental hypothermia in major trauma – A prospective observational study.
Resuscitation 2010 Nov.
3. Martin RS, Kilgo PD, Miller PR, Hoth JJ, Meredith JW, Chang MC. Injury associated
hypothermia: an analysis of the 2004 National Trauma Data Bank. Shock. 2005 Aug;
24(2):114-8.
4. Tisherman SA. Hypothermia and injury. Curr Opin Crit Care 2004; 10: 512–519.
5. Robinson S, Benton, G. Warmed blankets: an intervention to promote comfort to elderly
hospitalized patients. Geriatric nursing 2002; 23:320-323.
6. Durrer B, Brugger H, Syme D. The medical on site treatment of hypothermia. In:
Elsensohn F (editor), Consensus Guidelines on Mountain Emergency Medicine and Risk
Reduction. 1. ed. Italy: Casa editrice stefanoni – lecco, 2001, pp 71-75.
7. State of Alaska cold injuries guidelines. In: Alaska Emergency Medical Services
Program, Department of Health and Social Services, Juneau. Available at
http://www.chems.alaska.gov/EMS/documents/AKColdInj2005.pdf. Accessed 23 January
2012.
8. Giesbrecht GG, Bristow GK, Uin A, Ready AE, Jones RA.Effectiveness of three field
treatments for induced mild (33.0°c) hypothermia. J Appl Physiol 1987; 63: 2375-79.
9. Hultzer M, Xu X, Marrao C, Bristow GK, Chochinov A, Giesbrecht GG. Pre-hospital
torso-warmingmodalities for severe hypothermia: a comparative study using a human
model. Can J Emerg Med. 2005;7:378–386.
10. Kober A. Effectiveness of resistive heating compared with passive warming in treating
hypothermia associated with minor trauma: a randomized trial. Mayo Clin Proc. 2001. 76:
369-375.
11. Watts DD. The utility of traditional prehospital interventions in maintaining thermostasis.
Prehosp Emerg Care. 1999. 3:115-122.
12. Lundgren JP, Henriksson O, Naredi P, Bjornstig U. The Effect of Active Warming in
Prehospital Trauma Care during Road and Air Ambulance Transportation – a Clinical
Randomized Trial. Scand J Trauma Resusc Emerg Med. 2011 Oct 21;19:59
8

13. Lundgren JP, Henriksson O, Pretorius T, Cahill F, Bristow G, Chochinov A, Pretorius A,
Bjornstig U, Giesbrecht GG. Field Torso Warming Modalities: A Comparative Study
Using a Human Model. Prehosp Emerg Care 2009; 3: 371-378.
14. Parsons KC. Thermal comfort.. In: Parsons KC. Human thermal environments: the effects
of hot, moderate, and cold environments on human health, comfort and performance. 2.
ed. London, UK: Taylor & Francis; 2003, pp 91-130.
15. BS EN ISO 10551, 2001. Ergonomics of the thermal environment - Assessment of the
influence of the thermal environment using subjective judgement scales, Geneva:
International Standards Organization.
9

10

Cold Discomfort
Active
Passive
11

Validity and Reliability of the Cold Discomfort Scale - a Subjective
Judgement Scale for Assesssment of the Thermal State of Patients in a Cold
Environmemt
Peter Lundgren MD¹; Otto Henriksson MD¹; Kalev Kuklane PhD²; Ingvar Holmér PhD²;
Peter Naredi MD PhD¹; Ulf Björnstig MD PhD¹
1. Division of Surgery, Department of Surgery and Perioperative Sciences,
Umeå University, Sweden
2. The Thermal Environment Laboratory, Division of Ergonomics and Aerosol Technology,
Department of Design Sciences, Faculty of Engineering, Lund University, Sweden
Running head: Validity and Reliability of the Cold Discomfort Scale
Corresponding author: Dr. Peter Lundgren
Division of Surgery,
Department of Surgery and Perioperative Sciences
SE-90185 Umeå, Sweden
E-mail: peter.lundgren@surgery.umu.se
Telephone: +46706678316
Fax: +4690771755
E-mail for all authors: peter.lundgren@surgery.umu.se
otto.henriksson@surgery.umu.se
kalev.kuklane@design.lth.se
ingvar.holmer@design.lth.se
Conflicts of Interest: None declared
peter.naredi@surgery.umu.se
ulf.bjornstig@surgery.umu.se
Supported by: The National Board of Health and Welfare, Sweden
Key words: hypothermia, prehospital trauma care, emergency medical services, reliability,
validity, subjective judgement scale, thermal comfort
1

Abstract
Introduction: Alternative measures for assessmentof the thermoregulatory state of the
patient, such as subjective judgement scales for assessment of personal thermal state, might
be of considerable importance in field rescue scenarios, where objective measures such as
body core temperature and skin temprature, as well as oxygen consumption are hard to obtain.
Objective: To evaluate the Cold Dicomfort Scale (CDS), a subjective judgement scale for
assessment of the thermal states of patients in a cold environment, regarding reliability,
defined as as test-retest stability and criterion validity, defined as the ability to detect changes
in cumulative cold stress.
Methods: Twentytwo healty subjects performed two consecutive trials (test-retest).They
dressed in light clothing and stayed in a climatic chamber set to - 20º C for 60 minutes. CDS
ratings were obtained every five minutes.
Results: Reliability was analyzed by test-retest stability, using weighted kappa coefficient,
and was 0.84, including all the measurements made every five minutes and was 0.48 to 0.86
separated for every single measurement. Criterion validity, defined as sensitivity to detect a
difference in cumulative cold stress over a 30 minutes interval, was analyzed by comparing
median CDS ratings for a moving 30 minutes interval, which revealed that CDS ratings were
significantly increased during each 30 minutes interval (p < 0.001).
Conclusion: In a prehospital scenario subjective judgement scales might be a valuable
measure for assessment of the thermal state of concious patients. The results of this study
indicated that the CDS is both reliable and valid for such purpose.
2

Introduction
Admission hypothermia is an independent risk factor associated with worse outcome and
higher mortality in trauma patients (1 – 6) and initial actions to reduce cold exposure and
prevent further heat loss is an important and integrated part of prehospital primary care (7 –
11). The cold induced stress response will also render great thermal discomfort, which might
increase the experience of pain and anxiety even in still normothermic patients (12 - 16).
Conseqeuently, as part of primary medical care, it is important to have accurate measures to
evaluate the thermoregulatory state of the patient, both upon arrival of the rescue team and
during treatment and evacuation. In the field, especially in harsh ambient conditions this is
often hard to achieve. Measuring body core temperature as well as skin temperature might be
difficult (9) and measuring oxygen consumption for assessment of shivering is, in most
clinical scenarios, not possible.
Thus, alternative measures such as subjective judgement scales for assessment of the thermal
state of the patient might be of considerable importance in such scenarios both for an initial
assessment and for evaluation of the treatment provided. It is of utmost importanance that
those subjective judgement scales are reliable and valid.
Reliabilility refers to a measure´s lack of errors of measurement. (17) Validity can be divided
into content, construct and criterion validity, where criterion validity refers to a measure´s
association with one or more outcome criteria.
The most common single item judgement scales are Visual Analouge Scales (VAS),
Numerical Rating Scales (NRS) and Verbal Rating Scales (VRS). A VAS consists of a visual
line, usually 100 mm long, where the ends of that line are labeled with descriptions for the
extremes of the studied modality. The respondent places a mark on the line representing his or
her level of experienced intensity in relation to the described extremes. Instead of a visual
line, a NRS consists of a range of numbers, usually 0 – 10, and a VRS consists of a list of
words or phrases, describing various degrees of the studied modality (17). In clinical practice
such scales are commonly used for the assessment of pain, where they have been shown to be
both valid and reliable (17, 18). However, they are also used for the assessment of other
modalities such as thermal sensation and (dis)comfort (12 - 16).
The international standard BS EN ISO 10551:2001 outlays general principles for construction
of subjective judgement scales for assessment of the influence of the thermal environment
(19). There are however, to the authors’ knowledge, no previous studies on reliability and
3

validity of psychometric methods for assessment of the influence of the thermal environment
in more extreme ambient conditions.
In accordance with the basic principles stated in the international standard (19) and with some
modifications to increase usefulness in a prehospital rescue scenario we have designed a NRS,
the Cold Discomfort Scale (CDS), for assessment of the thermal state of patients in a cold
environment (20, 21). The objective of this study was to evaluate this NRS regarding
reliability, defined as test-retest stability and criterion validity defined as the ability to detect
changes in cold discomfort due to cumulative cold stress.
Methods
Design, settings and subjects
The study was conducted in October and November 2011 at the Thermal Environment
Laboratory, Lund University, Sweden. Thirteen male and nine female volunteered for
participation (Table 1). They were cardiopulmonary healthy and had no regular medication or
history of local cold injuries. None of them were habitual smoker or abused narcotics. Written
informed consent was obtained from all subjects. Ethical approval was obtained from the
Regional Ethical Review Board in Umea.
The study protocol was designed as a test-retest where subjects were exposed in -20 °C for 60
minutes to evaluate reliability and validity of the Cold Discomfort Scale. All subjects thus
conducted two identical trials on two separate occasions, at about the same time of day,
approximately one week apart. During the twenty-four hour period prior to the trials subjects
avoided smoking or drinking alcohol, had a minimum of six hours of rest during the night and
were also instructed to avoid physical exertions. The diet was not modified but they all had
regular meals.
Monitoring
Cold discomfort was monitored using the Cold Discomfort Scale (CDS), a numerical rating
scale, where the subjects assess the thermal state of their whole body, not specific body parts,
providing integer values from 0 to 10, where 0 indicates not being cold at all and 10 indicates
unbearable cold. Subjects were asked the following qestion:
On a scale from 0 to 10, where 0 means not being cold at all and 10 means unbearably cold:
How cold do you feel right now?
4

To assure that there was no risk of local cold injuries, finger and toe temperature was
continuously monitored using thermistors (Rhopoint Components Ltd, UK, accuracy ±0.2 °C,
time constant 10 s) taped to the left ring finger and the left index toe.
Ambient air temperature was continuously monitored using three sensors (PT 100, Pico
Technology Ltd, UK, accuracy ± 0.03 °C) positioned in level with the supine subject, adjacent
to the ankles, the mid trunk and the head.
Protocol
Subjects dressed in light, two-piece thermal underwear, a fleece cap, two pair of gloves and
two pair of woollen socks and an outer foot cover. Insulation of hands and feet were
reinforced to avoid the risk of local cold injuries. At first, subjects sat quitely at an ambient
temperature of about 21 °C for fifteen minutes of baseline data collection. They then entered
the climatic chamber (2.4 x 2.4 x 2.4 m), set to -20°C, and lay down in a supine position on a
foam mattress. One of the medical doctors responsible for the study (P.L or O.H)
accompanied the subject in the cold chamber during the whole trial, and every five minutes
subjects were asked to express their thermal state according to the CDS. After 60 minutes of
cold exposure the trial was completed and subjects exited the cold chamber. Clothing and
monitoring equipment were removed and the subjects then had a warm shower until
restoration of thermal comfort.
Data analysis
As the Cold Discomfort Scale comprises ordinal data non parametric statistics were used.
Reliability of the Cold Discomfort Scale was analyzed for test-retest stability, using weighted
(quadratic difference) kappa coefficient (22), comparing median CDS ratings between the two
trials, including all the measurements made every five minutes and also, separately for every
single measurement. StatXact 9 software (Cytel inc., Cambridge, MA, USA) was used for the
analysis.
Pre-study calculations indicated a minimal sample size of 18 to detect a difference in CDS
ratings of 2 or more (IQR; 2) presupposed 80% statistical power at an -level of 0.05.
Criterion validity was analyzed by comparing median CDS ratings over a moving 30 minutes
interval (5-35 minutes; 10-40 minutes; 15-45 minutes.etc) using Wilcoxon Signed Ranks test.
Statistical significance was defined as p < 0.05 and, in analysis of criterion validity, after
correction for multiple comparisons according to Bonferroni as p < 0.008. SPSS 18.0
software (SPSS inc., Chicago, IL, USA) was used for the analysis.
5

Results
Totally, 44 trials were conducted, each of the 22 subjects conducting two trials each. The
average ambient air temperature for all trials was ± °C (mean ± SD) and the average wind
speed was 0.2 ± 0.0 m/s (mean ± SD). Skin temperature of the left index finger and left
second toe never went below + 8 °C for any of the subjects.
Median CDS ratings increased from 0 (interquartile range, IQR; 0 - 0) during baseline to 7
(IQR; 5 - 7) at the end of the first trial (test) and from 0 (IQR; 0 – 0) to 6 (IQR; 5 - 7) during
the second trial (retest) (Figure 1).
Reliability analyzed for test-retest stability, using weighted kappa coefficient, were 0.84,
including all the measurements made every five minutes and were from 0.48 to 0.86 separated
for every single measurement (Table 2).
Criterion validity analyzed by comparing median CDS ratings (n = 22) over a moving 30
minutes interval revealed that CDS ratings were significantly increased during each 30
minutes interval (5-35 minutes; 10-40 minutes;15-45 minutes etc) (Table 3).
Discussion
Overview
In a laboratory setting the test-retest stability of median CDS ratings over the 60 minutes of
cold exposure was 0.84 (very good agreement) when all the measurements made every five
minutes were included and 0.48 – 0.86 (moderate to very good agreement) when separated for
every single measurement (22). The Cold Discomfort Scale was significantly sensitive to
detect a difference in cumulative cold stress over a moving 30 minutes interval throughout the
whole 60 minutes of cold exposure.
Reliability
It is always difficult to achieve identical conditions in a test-retest design when measuring
subjective parameters. Even if all arrangements are made the same, the subject might react
differently to the same cold exposure at two different occasions. There might also be a
habituation which can either increase or decrease the sensitivity to the exposure. CDS ratings
were generally somewhat higher in the first trial compared to the second trial, and this
difference might be a result of a decreased sensitivity to the cold exposure from beeing more
6

experienced and therefore less anxious about being exposed to the cold the second time
compared to the first time. However, test-retest stability was still very good when all the
measurements every five minutes were included and moderate to very good when separated
for every single measurement.
Validity
Criterion validity was defined as the minimum time for which a clinically significant
cumulative cold stress in - 20°C wind still conditions would be desirable to detect. The results
revealed that CDS ratings were statistically significantly increased for each 30 minutes
interval (5-35 minutes; 10-50 minutes; 15-45 minutes etc) and thus the cold discomfort scale
was valid for detecting a change based on the defined cumulative cold stress. However,
during the last 20 minutes it seems like CDS ratings are not increasing as much as during the
first 40 minutes. This might be an indication of a limitation to detect differences in cumulative
cold stress when cold exposure is protracted due to habituation to ambient conditions.
Practical implications
As part of primary medical care it is important to have accurate measures to evaluate the
thermoregulatory state of the patient, both upon arrival of the rescue team and during
treatment and evacuation. In the field however, especially in harsh ambient conditions,
reliable measures of body core temperature, skin temperature or shivering thermogenesis is
often hard to achieve (9). In a prehospital rescue scenario, subjective judgement scales might
thus be an important alternative tool for assessment of the thermal state of patients in a cold
environment.
Assessment of the thermal state of the patient might be an early predictor of cold stress and
therefore may be used to evaluate the risk of developing hypothermia. It is also important not
to underestimate evaluation of the patient’s subjective experience of medical care. Much
resources and a lot of effort are invested to optimize medical treatment including pain relief
but patients thermal comfort is easily forgotten. Therefore, reliable and valid subjective
judgement scales for assessment of the thermal state of patients in a cold environment is
important for improving prehospital medical care.
The international standard BS EN ISO 10551:2001 outlays general principles for construction
of subjective judgement scales for assessment of the influence of the thermal environment
(19). These general principles recommend symmetrical 7 to 9-degree rating scales comprising
a central indifference point and two times 3 or 4 degrees of increasing intensity for both hot
7

and cold. Because subjective judgement scales used in prehospital as well as hospital medical
care most commonly ranges from 0 to 10, for example when assessing pain intensity using the
Visual Analouge Scale (VAS), we considered a similar range of the CDS would be more
easily understood by patients and also very important, more familiar to the rescue personel. If
judgement scales with different ranges are used, that could be confusing. Furthermore, since
we are only interested in cold exposure, we think it is better to simplify the scale to be
assymetrical, describing only cold. In the litterature (19, 23) there is a distinction between
perception/ thermal sensation and affective assessment/ (dis)comfort where the former
describes perception of afferent perpiheral and central stimuli and the latter describes the state
of mind that expresses (dis)satisfaction with the surrounding environment. The CDS does not
differentiate between perception and affective assessment of the thermal environment. This
design enables rescue personel to give short, pithy instructions to patients when obtaining data
instead of explaining the different definitions of perception versus affective assessment.
Making these modifications to international standard instructions we think that the CDS has
advantages in practical use in a prehospital rescue scenario.
Limitations and further research
Subjective judgement scales as a tool for assessment of the thermal state of the patient is of
course limited to conscious patients, not suffering from any major distracting injury. To the
authors’ knowledge this is the first study evaluating reliability and criterion validity of a
subjective judgement scale for assessment of the thermal state of patients in an extreme cold
environment. Considering the small study population, limited time period for cold exposure
and limited ambient conditions further studies to confirm these results may be required.
Conclusion
In a prehospital rescue scenario subjective judgement scales might be a valuable measure for
assessment of the thermal state of concious patients. The results of this study indicated that
the CDS is both reliable and valid for such purpose.
8

Acknowledgements
The study was supported by the National Board of Health and Welfare, Sweden.
9

References
1. Ireland S, Endacott R, Cameron P, Fitzgerald M, Paul E: The incidence and significance
of accidental hypothermia in major trauma - A prospective observational study.
Resuscitation 2011, 82(3):300-306.
2. Beilman GJ, Blondet JJ, Nelson TR, Nathens AB, Moore FA, Rhee P, Puyana JC, Moore
EE, Cohn SM: Early hypothermia in severly injured trauma patients is a significant risk
factor for multiple organ dysfunction syndrome but not mortality. Ann Surg 2009,
249(5):845-50.
3. Martin RS, Kilgo PD, Miller PR, Hoth JJ, Meredith JW, Chang MC: Injury associated
hypothermia: an analysis of the 2004 National Trauma Data Bank. Shock 2005,
24(2):114-8.
4. Wang HE, Callaway CW, Peitzman AB, Tisherman SA: Admission hypothermia and
outcome after major trauma. Critical Care Medicine 2005, 33(6):1296-301.
5. Shafi S, Elliott AC, Gentilello L: Is hypothermia simply a marker of shock and injury
severity or an independent risk factor for mortality in trauma patients? Analysis of a large
national trauma registry. Journal of Trauma Injury Infection & Critical Care 2005,
59(5):1081-5.
6. Gentilello LM, Jurkovich GJ, Stark MS, Hassantash SA, O’Keefe GE: Is hypothermia in
the victim of major trauma protective or harmful? A randomized prospective study.
Annals of Surgery 1997, 226(4):439-47.
7. Danzl DF: Accidental Hypothermia. In: Wilderness medicine. Edited by: Auerbach P.
Philadelphia. PA: Mosby Elsevier; 5th edition 2007:125-159.
8. Giesbrecht GG, Wilkerson JA: Hypothermia frostbite and other cold injuries prevention,
survival, rescue and treatment. Seattle, WA: Mountaineers Books; 2nd edition, 2006.
9. Socialstyrelsen: Hypothermia: cold injuries and cold water near drowning.
Stockholm, Sverige: The National Board of Health and Welfare (Socialstyrelsen); 2nd
edition 2002.
10. Durrer B, Brugger H, Syme D: The medical on site treatment of hypothermia. In
Consensus Guidelines on Mountain Emergency Medicine and Risk Reduction. Edited by:
Elsensohn F. Italy: Casa editrice stefanoni - lecco; 1st edition 2001:71-75.
11. State of Alaska cold injuries guidelines: Alaska Emergency Medical Services Program,
Department of Health and Social Services, Juneau.
10

[http://www.chems.alaska.gov/EMS/documents/AKColdInj2005.pdf], Accessed 10
January 2012
12. Robinson S, Benton G: Warmed blankets: an intervention to promote comfort to elderly
hospitalized patients. Geriatric nursing 2002, 23:320-323.
13. Grant SJ, Dowsett D, Hutchison C, Newell J, Connor T, Grant P, Watt M. A comparison
of mountain rescue casualty bags in a cold, windy environment. Wilderness Environ Med.
2002 Spring;13(1):36-44.
14. Kober A: Effectiveness of resistive heating compared with passive warming in treating
hypothermia associated with minor trauma: a randomized trial. Mayo Clin Proc 2001,
76:369-375.
15. Thomassen et al.: Comparison of three different prehospital wrapping methods for
preventing hypothermia – a crossover study in humans. Scandinavian Journal of Trauma,
Resuscitation and Emergency Medicine 2011 19:41.
16. Lundgren P, Henriksson O, Naredi P, Bjornstig U: The effect of active warming in
prehospital trauma care during road and air ambulance transportation - a clinical
randomized trial. Scandinavian Journal of Trauma, Resuscitation and Emergency
Medicine 2011 19:59.
17. Jensen MP: The validity and reliability of pain measures in adults with cancer. Pain.
2003;4(1):2-21. Review.
18. Williamson A, Hoggart B: Pain: A review of three commonly used pain rating scales. J
Clin Nurs. 2005 Aug;14(7):798-804.
19. BS EN ISO 10551, 2001. Ergonomics of the thermal environment - Assessment of the
influence of the thermal environment using subjective judgement scales, Geneva:
International Standards Organization.
20. Lundgren P, Henriksson O, Naredi P, Bjornstig U: The effect of active warming in
prehospital trauma care during road and air ambulance transportation - a clinical
randomized trial. Scandinavian Journal of Trauma, Resuscitation and Emergency
Medicine 2011 19:59.
21, Lundgren P, Henriksson O, Naredi P, Bjornstig U. The Effect of Active External
Warming on Cold Discomfort in Field Treatment of Trauma Patients – a Clinical
Randomized Trial. Mnuscript.
22. Altman DG. Practical statistics for medical research. London: Chapman and Hall; 1991.
23. Flouris AD. Functional architecture of behavioural thermoregulation. Eur J Appl Physiol
(2011) 111:1-8. Review.
11

12
Table 1. Subject characteristics.
Gender
(m/f)
13/9
Age (years) 23.3 (4.4)
Weight (kg) 72.7 (15.3)
Height (cm) 178.9 (9.6)
BMI 22.5 (2.9)
Values are mean (± SD) or number of
subjects.

Table 2. Test-retest stability CDS ratings at 5 minute intervals.
Time (min)
Test *
(n=22)
Re-test *
(n=22)
Weighted kappa coefficient **
(n=22)
5 2 (1.25 - 3) 1 (1 - 2) 0.56 (0.25 - 0.86)
10 3 (2 - 3) 2 (1 - 2) 0.48 (0.20 - 0.77)
15 3.50 (3 - 4) 2 (1.25 - 3.75) 0.56 (0.31 - 0.81)
20 4 (3.25 - 4) 3 (2 - 4) 0.60 (0.38 - 0.83)
25 5 (4 - 5) 3 (2.25 - 4.75) 0.53 (0.30 - 0.76)
30 5 (4 - 6) 4 (3 - 5) 0.68 (0.48 - 0.87)
35 6 (4 - 6) 4 (3 - 5) 0.64 (0.40 - 0.88)
40 6 (4 - 6) 4.5 (6 - 4) 0.70 (0.49 - 0.90)
45 6 (4.25 - 6) 4.5 (6 - 4) 0.72 (0.51 - 0.92)
50 6 (5 - 7) 5.5 (5 - 7) 0.76 (0.57 - 0.96)
55 6 (5.25 - 7) 6 (5 - 7) 0.86 (0.72 - 1.0)
60 6.5 (5.25 - 7) 6 (5 - 7) 0.85 (0.81 - 0.99)
Values are median (IQR)* and weighted kappa coefficient (95% CI)**.
13

Table 3. CDS change over a 30 minutes moving interval.
Time (min) Test and retest (n=22)* Test and retest (n=22)**
Wilcoxon signed
rank test
5 vs. 35 2 (1 - 2.25) 5 (3.75 - 6) p < 0.001
10 vs. 40 2 (2 - 3) 5.5 (4 - 6) p < 0.001
15 vs. 45 3 (2 - 4) 6 (4 - 7) p < 0.001
20 vs. 50 4 (2 – 4) 6 (5 - 7) p < 0.001
25 vs. 55 4 (3 - 5) 6 (5 - 7) p < 0.001
30 vs. 60 5 (3 - 6) 6 (5 - 7) p < 0.001
Values are median (IQR).
* First time in interval, ** second time in interval.
14

10
9
8
7
6
5
4
3
2
1
0
0 10 20 30 40 50 60
Figure 1. Median CDS ratings of test (n = 22), retest (n = 22) and merged median CDS ratings
of test-retest (n = 22).
15